you are definitely not doing anything wrong - it appears to be very common (happened with all my NCT friends) at 4/5 months that babies who have previously slept through start waking more- breastfed and bottle fed so nothing to do with that. It probably is partly hunger - the small bits of food he will be eating aren't going to fill him up as much as milk, make sure he gets enough milk.....perhaps a big feed at 10/11 - I introduced a bottle from DP at this time, so I could get a block of sleep - its bloody exhausting though!
I think - just my theory! - that at this point they are just more wakeful, they have come out of a phase where they were sleeping deeply and now they are like adults in a way, they wake a lot at night and just need to learn how to go back to sleep .....I know it seems a long time, but if you keep letting him settle himself (and make sure he has enough milk of course) he will gradually start sleeping longer stretches again -
my DS took until about 7/8 months to get back to normal.....but...I think I made it worse by continually feeding him back to sleep - I'm not saying that is wrong, do whatever you think is right, but I was overdoing it! but then again - you have to do what gets you the most sleep! they do grow out of it.
putting him in his own room was v helpful but we did wait untli 7 months - next time I would pribably do earlier - I would rather sleep on floor next to his cot than have him in our room waking so often -
advice we had from HV was wait until they are properly established on solids - not little pieces, but actually eating meals - then give a dream feed at about half 10 and try to push them through till morning with patting/ stroking in cot - I have to admit I felt that was too harsh but I did find the patting/ not picking up too often from the cot bit v useful in helping him fall asleep - picking him up when he was tired got him really upset, now a few pats and he rolls right back to sleep most of the time.
we did keep going with the patting/ shhing and got him further and further through without much upset - so, first I aimed to get him to sleep till 3 - then till 5 - then he started sleeping till 7 on his own - recently.
sorry, what a long post! - anyway, you have my sympathies - its very normal, its a developmental phase, and just keep doing whatever you need to sleep as a family, they do grow out of it once eating proper food