Are you swaddling her? I swaddled all three of my kids up until at least 6 months. I used a tight wrap (Happiest-Baby-on-the-Block style) with a large, non-stretchy blanket and then put a secured wrap (such as Loving Baby from onestepahead.com or a Swaddle Me blanket) around that to keep them contained once they got strong enough to break out of the blanket alone. Swaddling added hours to sleep for all of my kids.
Also, my middle son was much like your daughter in terms of sleep when he was 4 months old. I would have to rock him for about an hour to get him to sleep soundly enough that I could put him down for a nap, and by that time I didn't have much of my older son's naptime left. I work part-time from home and counted on naptime to get work done, so I had to do something. I had used the cry-it-out method successfully with my older son, but not until he was about 7 months old (he was a great sleeper, anyway). I thought 4 months was probably too young, but I was desperate, so I tried. The first afternoon he cried on and off (while I went in to check on him every few minutes) for an hour. I felt like a terrible mom and was sure he had no idea what I was asking him to do. BUT, day two, he was asleep in 15 min. By the end of the week, he was napping on his own, with minimal fussing.
I didn't apply this method to nighttime until he was more than 1, by which time I was severely sleep deprived from the multiple middle-of-the-night feedings. At 4 months, babies still need night feedings, but I wish I'd done something about the frequent night feedings sooner than I did. For now, though, I'd just try swaddling your daughter at night if you're not doing that already.
I know a lot of people don't like cry-it-out, for various reasons. But I have used it with all three of my kids, who are now 5, 3, and 18 months, and all three of them are excellent sleepers now. Even the 5-year-old will still take a nap -- voluntarily. They do not wake us up in the middle of the night. They know how to put themselves back to sleep. I felt like a bad mom for a few days with each one, but three days out of a lifetime is not much, and now I've given them (and their parents) the gift of good sleep. I think it's worth it.