I can completely relate to how overwhelmed you are feeling- I, too, have a 4 yo, a 2 yo (gotta loves those wonderful two's huh!!!), and a 4 month old. I can honestly tell you that it will get easier. Those first two months or so were a really stressful, crazy, chaotic time as everyone tried to readjust to our new family addition. Lack of sleep and trying to breastfeed and taking care of a toddler and a preschooler was enough to drive me over the edge!!!
What I can tell you is to try to recruit any help you can- whether it is a trusted friend or family member, to give you a little break (take a shower, run to the store, go for a walk- anything to give you a few moments of peace and quiet and time to regroup.). Sometimes that few moments of down time is my saving grace!!!!!!
It will get easier as your little one grows. And you'll be amazed by way those older siblings will keep the baby distracted!!! My LO smiles and laughs and gets so excited when she sees her big sisters now- it just warms your heart to see how happy the three of them can be!!! And getting the older ones to "help" by grabbing burp cloths and diapers keeps them busy and lends me a much needed hand!!!
Don't get me wrong- we still hit some rough patches in the road- but you will get through them!!!! Just keep your chin up!! You can do it!!!!!!!