Teething can start anywhere from about three months and onward and the pain can start long before a tooth pokes through. I found that the worst was the first tooth with my little girl...who is now 13 months. She was acting fussy long before I ever saw a tooth, and I had suspicions that might be it.
Or your babe might be overtired which can be a vicious cycle...make sure she gets two good naps or at least an hour each any way you can!! And an early bedtime can actually be better and lead to longer/better sleep in the long run. She might be telling you its too late for her...
Its also a normal thing for a baby to start waking a lot more around 4 months. Maybe your LO is starting a bit early! Google: 4 month wakeful period. Its a developmental thing and most of the time it passes soon. The important thing at this age is to not create any bad habits or dependencies for sleep! Like I did! My LO slept great until around 4 months...then she suddenly didn't anymore. And I got into a habit of rocking her or nursing her to sleep, putting her into the crib fully asleep every time! And she woke up every hour or two all night long for months! And slept in our bed a lot... which I still have mixed opinions about. So she never learned how to fall asleep on her own until almost a year...and then I had to break some major sleep dependency issues which sucked for a few days until she got it! Now she sleeps better again, but only after I taught her to fall asleep on her own.
Gentle sleep coaching can start as early as around 4-5 months...where you try to put your baby down awake to fall asleep. Its often a lot easier than waiting until baby is older and more stubborn. So first if you don't already do this... make a soothing bedtime routine that is very consistent. We do a bath, pjs on, nursing/drink, 3 books, sing songs, and then lights out. Start this routine every night and do it for a few weeks. Also start bedtime at an early time. Usually for most babies 7-7:30 is the latest they should be up at that age. Its important to make sure baby is not too overtired...or they actually sleep worse! Then later when she is used to the routine you can try to put her in the crib drowsy but still awake. At a young age its better not to let baby cry too much. But if she seems ok with it, let her learn how to sleep! Good luck!