We're just coming through this with DS, and it's so hard! He wasn't a great sleeper after the very first few weeks, but had reached the stage of going 3hr stretches at night, when he reached 4 months and it all fell apart! He's now almost 6 months and things have improved again.
If you Google "4month sleep regression" you'll get info on how many babies often do this - their sleep changes at about that time, and initially it can make them wake up more. It seems to take a few weeks to a couple of months to settle down. I found that just knowing that made it easier to cope with!
In the meantime, here are some things that helped:
He would sleep in his pram during the day, so I put him in it at night. He seemed to feel more secure in there or something. I just used the pram top, on the floor, propped up well and with straps removed or pulled back as much as possible. This allowed him to sleep in our room (rather than in his cot in his own room), so at least it was easier for me to get to him for feeds.
He seemed a bit cold some nights, so I got him a couple of "gro-suits", which have padded arms to keep him warm.
He has eczema, and we realised that that could have been waking him up, along with teething. Couldn't do much about the teeth other than painkillers; took him to GP about eczema and got some creams etc which seemed to soothe it. Maybe have a think whether your DD has any kind of condition that could be keeping her awake. Even check things like the seams on sleepsuits in case something like that is digging into her.
I've never known how much daytime sleep affects things. My MIL insists that we should keep him awake so he sleeps at night, but I think he does better if he's well-rested. He doesn't really nap that much though, unless he's in the pram or out in the car, so I try to plan the day so that he'll get a sleep morning and afternoon at least.
Other than that, call in all the help you can get and sleep when you can!