What to do with a 4-hour layover in FRA airport?

Unless you know your way around the airport fairly well, or your flight is early and your next flight is an inter-Europe flight I don't think I would go into the city. I know my way around and could do this, but I know the most efficient ways to get from A to B. I do have lots of posts on this forum about Layovers in Frankfurt and how to do them, but 4 hours is a bit short. Subtract the time to get your carry-on stored and pick it up again, and you have even less time. Where is your flight arriving from and where are you going. This might be helpful to know. Check the history of your arrival flight. Is it often early or on time? If you are coming from N. America they are often early, but some airports seem to always have delays.

The Forbes layover post is more than several years old, judging from the ticket prices. A one way ticket is now 4.35 € and an all day ticket is 8.50 €

Have a look at this comprehensive post about layovers in Frankfurt, but due to it being written over a year ago, the ticket prices are now outdated.
http://www.tripadvisor.ca/ShowTopic-g187337-i301-k6809373-Layovers_in_Frankfurt_how_to_and_what_to_see-Frankfurt_Hesse.html

Thanks for the helpful information and links --
I will need to factor in both the time needed at the airport and the amount of stuff I'm carrying.
I'm no expert about the FRA airport but having gone through it a couple of times I know enough
to want to get away from it even if only for an hour or two to go into town and stroll and get a snack.

3 stops on the S-Bahn sounds pretty appealing compared to the outsized metal fixtures and stale pastries
at the airport!

If you do end up being stuck there, due to flight delays, etc. the pastries at Marche' Cafe' and Kemps Bakery are very fresh and made on site. Really delicious. Marche' makes fresh food all the time. They are located next to the Meeting Point in terminal 1.

It usually takes about 30-45 min. to emerge into the airport. Are you arriving and departing from Terminal 1? If it is Terminal 2, you will need to add about 10 min. more to get to the Regional train station.

This particular story had a pleasant surprise ending -- when I got my bearings at FRA and started plotting my brief escape on the S-bahn, it turned out there was an earlier direct flight starting to board, so I hustled over to that gate and the agents were nice enough not only to move me onto that flight but to also give me an economy-plus ticket, so there was less risk of a legroom incident :P
So, I missed my short stroll around Frankfurt, but I gained a daylight landing back in the US.

Brad, I've certainly noticed how the major American carriers have lost sight of customer service in their pursuit of the bottom line -- getting them to change flights same day has become harder and harder.
This particular flight was a code share between United and Lufthansa,
and the Lufthansa desk said they would make the change if they could,
but the ticket had been issued on the United number,
so when I went to the United desk, I told them that Lufthansa would make the change if they could,
but it was up to you -- there was a little behind the counter consultation about the coding involved
in their system, and then they went ahead and moved me to the flight that was boarding.

This was exceptional, for these days --
I remember in years past, though, when I was doing a lot of LAX to SFO hops,
the major carriers would not hesitate to move me to whatever flight was taking off when I got to the terminal...

Has anyone put together a collection web page for United Airlines horror stories?

I tweeted about the lost time and money I had when they caused me a missed connection in LHR,
after I spoke to three different agents pointing out the problem ahead of time,
and a United public relations person invited me to fill in their internal customer service form,
which is largely a way for them to get people to let off steam without it being made generally known.

United Airlines is like that old joke about the weather -- everyone complains about it but nobody ever does anything about it.