In reply to Derry:
Climbing in a four is very easy and not much slower than the pace of the slowest climber. I have done this for years with my kids on multi pitch bolted and trad. You just need to be organised.
Some tips:
1. set the belay using a cordelette or a sling with a powerpoint
2. belay using a Reverso in guide mode - anything else would not be safe.
3. either use 3 ropes as talked about above (and if you have 3 people who can belay), or use 2 ropes with one second on one rope and 2 on the other (one tied into the end, the other about 5 meters from the end).
4. The key question is where to put the weakest climber. It really depends on how weak they are. When Theo was very young (5yrs) he went on the bottom of the rope with two climbers with his mum just above. However this means that 2 climbers end up climbing at the pace of the weakest. This can spoil things a bit for the better climber, so you might like to put the slowest on the line by his/herself. This would not be best if they were nervous.
5. If the belays are bolted and you own two PAS-style daisies then consider using them rather than cordelettes at the belays by clipping the PAS as a big loop between the two bolts. This gives multiple clip-in points for people, reversos or sacks.
6. It will be easier if the best climber simply leads everything, however if you do want to swap leaders then equip everyone with a cow's tail so they are safe as you will need to untie and possible sort the ropes when they get a bit messed up at the swap.
7. A party of four is exactly that, a party. So take lots of food, cameras, water.
Enjoy.