What's the average nap time for a 4 month old? I know she's getting out of that newborn pattern where she's sleeping all the time, but I have no idea what's normal since my DS was a terrible sleeper and only took 15-30 minute catnaps from the time he was 2 or 3 months. I know it varies from baby to baby, but about how much sleep do they need?

At 4 months they should be napping around every 2 or so hours. The legnth of time they sleep can vary. DD will usually sleep for about 45 min-1 hour. DS was like your first...he only ever napped for 15-30 minutes...drove me nuts!

I'll tell you what worked like magic for me. Once my daughter was about 3 months old, I got her into a great nap schedule. She would go down 1 1/2 to 2 hours after she got up in the am and sleep for about 2 hours. Then she'd go down again 1 1/2 to 2 hours after she woke up from that nap for another 2 hours. Babies that age need 4-5 hours of napping sleep a day. Napping in the car or in a stroller doesn't give them the kind of restorative sleep that they really need. It was hard at first to stay home basically all morning so that she could sleep but once she got in her groove, it was SO worth it. Its all about consistency coming from her caregiver...everyday the same thing.
People tend to think that if your baby doesn't nap during the day, she'll sleep so great at night. That's really not true. Sleep begets sleep. The more sleep they get, the better their sleep is, the easier they go down and the longer they stay asleep. There is this book out there all about healthy sleep habits for kids, written by a sleep doctor. It changed motherhood for me!!! Try to find it!!! Good luck!