Janet, as usual, has lots of good info above. If the ND pass is too much hassle, you can buy packages (carnets) of 10 individual tickets good for Paris Metro and buses. They cost a little less than individual fares, save time, and you can split them up in your party. With three of your over 4+ days I'd get at least a couple of carnets to start.
The Paris Museum Pass would be a good deal, saving time and probably money. Not to be confused with the "Paris Pass," which is a different product and not generally considered a good deal.
With so little time you might want to skip Giverny, which is a long way out of town, and maybe even Versailles, unless you feel strongly about either of them. Paris itself has much more than enough for four full days. No need for guided tours, though you might want to check out Paris Walks to explore particular neighborhoods or themes. The hop-on-hop-off bus might be a good choice for your first day, to get an overview of the city, but these can be slow, crowded, and with audio guides that are hard to understand -- plus expensive. Consider some of the regular city buses for getting around -- slower than Metro but you see all the street stuff.
Finally, if you can't get up the Eiffel Tower, the Tour Montparnasse offers a similar view (one which includes the ET) for a lot less effort. Of course you'll miss the pickpockets and the guys selling cheap ET reproductions! ;-)