Infant Sleep Advice Needed/why the Change Now?

Babies are unpredicable little buggers, so I can't tell you why his sleep pattern changed. My only advice is that if he wakes hungry in the middle of the night to give him a bottle, either of expressed breast milk or formula, whichever you're doing. You don't want your body to get back in the habit of nursing at night, as it will be very uncomfortable when he stops again.

I also wouldn't try keeping him up if he's tired. A sleepy baby should sleep.

Good luck. Hopefully this passes fast.

I am thinking that a "dream feed" might be definitely worth trying. If he is falling asleep feeding then when he wakes he wants to fall back to sleep sucking...which could be why the pacey is working for you.
Check out www.sleepyplanet.com. Jill and Jen have wonderful advice and explain it in a simple and concise manner.
You probably already know this...breast milk is digested faster than formula so breast fed babies sometimes have a harder time making it through the entire night. I am not suggesting that you switch to formula I am just stating a possible reason for his waking. Good luck. www.parentingintheloop.com

Could be a growth spurt, at 4mos. My daughter (now 6mos) went through a phase where she was RAVENOUS for about a week. I remember my son doing the same thing at that stage.
If he's eating, feed him.
When he starts to not eat again, I would sleep train him. Sounds like you use the paci as a barometer for whether or not he's hungry. So keep with that, I think.
We did CIO at 22 weeks because we were playing that game all night long. She just wanted to hang out with us, and the paci wasn't working to placate her (and she wasn't eating when we tried to feed her). So for us, that worked wonders.

It seems like what you've been doing is working, you just have to adjust for that night waking. If he's not eating, I think it's time to do whatever sleep training method you're comfortable with. At this stage, they start to become aware of when you're in the room, and they're all for it!

He is 4 months so he could be teething or having a growth spurt. You could try a dream feed before you go to bed at night.

It seemed to me like as soon as I got used to my son's schedule, sure enough it would change. As he gets older, his body changes obviously, so you have to take this into account, and also one of these days (if not now), he will start teething which will most likely totally throw things off. Hang in there!

I wonder if it's not so much a pattern thing, as a comfort thing? At around four or five months, kids start to really notice the world around them, and become aware when things change. I wonder if he wasn't always sort of waking around the same time, and just falling back to sleep. (We all get close to waking several times a night. It's a sleep cycle thing.) Now that he's started to become aware that things are different, well, he fell asleep with the pacifier the first time, so when he woke in the night and didn't have his paci, he needed it again. He fell asleep eating (or, at least, the last thing he remembered before falling alseep was eating), so when he woke up, he wanted to be eating. It's sort of like waking up and finding your pillow has fallen off the bed. It's disorienting, and you want things to go back to how they were when you fell asleep. Could that be it?

Is skipping the paci to go to sleep, so you won't have to replace it in the night an option? Or try an activity between nursing and sleep, so that it's not the last thing he's thinking about when he falls to sleep. Sing to him or read a story or something, and let him go down drowsy but awake. If the last thing he's thinking of is being gently put in bed with his blankie, maybe just being in bed with his blankie (or whatever his lovey is) will be enough to soothe him back to sleep if he wakes again.