DS is 4 months too, and I'm interested in others' experiences.
He's bf and fed on demand which is basically about every 3 hours.
We have no real routine except in the mornings, when he feeds at about 7.30, sits in his bouncy chair while I have a shower and tidy up, and then is in the sling (asleep) from about 8.30 for an hour and a half while I walk the dog. His bath is after that, as he is hot after the walk.
When he looks tired, usually having been awake for 1.5 hours, I put him in bed awake, swaddled, with a soother and walk away. He's used to this now and will even sleep on other people's sofas etc depending on where we are. He only naps for about half an hour at a time.
I never worry about when I leave the house as it doesn't seem to matter. I suppose I try to have him recently fed as it gives me most time before I need to feed him again, but it's easy with bf to feed anywhere. The other day I was in a clothes shop and realised he was hungry, so I picked up a random garment, went to the fitting room and fed him there.
If he needs a nap when we're out, he'll sleep pretty well in the buggy or failing that I stick him in the sling.
He tends to not want a nap in the evenings and is horribly bright-eyed from about 6 to 9 or (aaargh!) 10pm, at which point he falls asleep and wakes usually just once in the night for a feed.
That all sounds very random and unscheduled!
DH had him last week as I was working, and kept a record of things. He took a bottle about every 3 hours and the amounts were so variable, from 50 to 150ml.