Help with accomodations in Croatia and Venice . . . guidance please

I can't speak to Venice, but in Croatia, it is quite common to rent apartments (for less than you would pay for a crummy hotel at home in most cases). These can be rented direct from owner or through an agency, and you'll find them on sites like booking.com as well. Quality is typically excellent--plus you can get a balcony with view and have a kitchen for some meals. That should make for a simpler process. I'd look for two bedroom apartments and put him on the couch, which I would certainly not mind if I was in his shoes, but you could also look for places with sofa beds (not uncommon).
In high summer, some places have three-night minimums, but in May-June it should be less of a concern.

Try the website booking.com . On that site, you can choose room types and see locations on a map. Many bookings offer free cancellation (up until a few days before arrival) so you can book a few things to get started but keep looking and cancel/change if you find something better later. Just read the cancellation policy carefully before booking - each property's policy is different.

We also recommend Booking.com. We recently stayed at Mescanka in Ljubljana. It's a lovely apartment on the river. You may be able to get an adjoining room to accommodate all of you. The facility was immaculate, reasonable & close to restaurants and sights. The staff were very helpful.
It's a popular place. We booked it about 10 months in advance.

maestrojohnnyj, its been 40 years so a lot has changed. You don't have to write, to see what's available. Most places have online booking in which you get to specify number of people, dates, etc. It will tell you if they have such a room. The online booking site is usually linked from the home page of the hotel you are looking at and almost always has one English language (British flag) version. So you don't have to directly contact each hotel to ask questions. Just browse online. What you need to know is that things work different over there, as hotels are required to charge by the number of persons in a room, not just adding a pullout bed. And you can't just show up and ask for an extra cot to be brought in. You may find it difficult to find four person rooms in most places. You may know these things, so I don't mean to be pedantic but I know it can be challenging and many infrequent travelers are not familiar.

When the 4 of us traveled as a family in Europe and my daughters were young adults we sometimes used 'family rooms' in hotels. They're typically the attic room in an older-style hotel. I'm not sure how old your daughter and her boyfriend are vs. if there are any restrictions on what constitutes a 'family' in terms of booking the room. Perhaps someone can weight in on that, or you can call a hotel if interested and ask. Generally we were much more comfortable in apartments, though.

I second using booking.com, but we have also had good luck with https://www.wimdu.com/ for apartments.

While there may be some hotels with a single room that will suit you (separate areas for you and your wife, and for your daughter and her boyfriend), it's much more likely that you'll need two rooms in a hotel, or an apartment.

Stan made a very important point that's worth emphasizing. In Europe, rooms will only hold the number of people stated. The beds and the rooms can be small, so they can't accommodate anyone else. It's not like in much of the US, where the room has two queen or two king beds, so adding an extra person at the last minute is easy.

If you do have any questions about whether a particular property has the kind of accommodations you want, you can email them directly to ask for more specifics.

The RS guidebooks frequently have at least one listing in the hotel section that has a QB (quad room). But since hotels in Europe typically charge by the person a QB will usually be significantly more expensive than a double. We've also booked two separate double rooms in an inexpensive hotel (e.g. Rick's single $ hotel listings) but at the moderate prices that we prefer those rooms are usually gently worn.

Our young adults have really enjoyed the extra space and privacy that an apartment affords. Apartments typically have a small seating area where everyone can chill. The biggest downside we've experienced is the time to takes to meet the apartment host if you have to get a key. We had a guy in Italy that was an hour late to meet us with the key, but that is not typical. Usually this does happen smoothly. I pay attention to the check in process and any comments in reviews before booking. Another upside to an apartment is that some will have a washing machine (dryers are much less common in my experience). So if you are in a location for multiple nights you can wash the first night and give the clothes plenty of time to dry. I usually plan a laundry stop for about midway through a two week trip. If we're just relaxing in the apartment it's easy to throw in a load as opposed to washing out clothes in the sink. This may facilitate packing light, which makes for easier travel, and relieves anxiety you may have about everyone's luggage fitting into the trunk of smaller European rental cars; been there! We now all take carry-on sized luggage and a daypack and even that can get tight in some cars with as few as three people.

Thank you, everyone, for your helpful suggestions.

I think I've found accommodations that will be suitable - separate room for my wife and myself, a room for my daughter, and a sleeper sofa or bed in a separate room or living area for the boyfriend. I used booking.com and airbnb. - jj

I'm probably the opposite. Booking.com doesn't work well when you don't want the standard double room. My experience has been better calling hotels directly and asking what they would suggest. When I traveled with my family, it wasn't unusual to stay in a suite on occasion, two hotel rooms sometimes and small apartments (owned by the hotel) other nights. I'd call and tell them what I needed, for example, "a room for three nights for five, myself, spouse, and three kids 2 boys, ages 15 and 12, and an 8 year old girl." I'd let them tell me what they had that would fit us. If it sounded good, I'd ask the price. If the price was good, I'd book it.

Also I've had issues, that were their fault, showing up with both booking.com and hotels.com reservations. Hotels.com reservations seem to get me in the worst room the place has to offer. The last straw was a room without hot water in Beaune. Booking.com switches dates (it happened to another person I know, so it's not just me) when you search by location - even though you started with a date. I've decided I'm not using them anymore. If you do, pay extra close attention to every detail before you book.