Ah, no-one's posted my favourite links on the 4 month sleep regression: here, here, here and here.
Often better naps do = better sleep, but IME it's often to do with the general trend rather than one day having an effect on the following night. Insufficient naps = overtiredness = high levels of stress hormone (cortisol) = lots of adrenaline produced = hard to go to sleep & stay asleep.
But lots of babies around 4 months or so only have 30-40 min catnaps whatever you do, and are constantly overtired. Many grow into longer naps at some point.
Babies' biological clocks are really just developing around 4 months, so they're only just getting into a more predictable napping pattern then - usually 3 or 4 naps a day if they take some longer ones. Being awake for around 90 mins at a time before needing another sleep is totally normal.
I found Healthy Sleep Habits, Happy Child to be a great book if you realise that (a) he's mental to advocate 'leave them to scream' as a sleep training method and (b) you realise that his approach doesn't necessarily work for your baby (although he says it works for them all). As it happens, it does for my DS at the moment - but it didn't use to suit him.
Strawberrycake, enjoy that - you don't realise how lucky you are. My 8.5 m.o. DS has never dropped off like that, even when he was a newborn.
On the needing solids to sleep better, 8.5 m.o. DS still needs a night feed although he has bf 6 or 7 times a day and eats so much in solids that one of his meals is often half of what my DH would eat (and DH doesn't have a small appetite!)
IME sleep often gets worse when you start weaning anyway, as they have to get used to food going through them.
Hang in there, it does pass. During the 4 month regression my DS was waking 3 or 4 times a night instead of once or twice. By 5 months he'd gone back to 1 or 2 - I didn't do anything different, just fed him every time he woke. Starting co-sleeping saved my sanity.