Please can you post your EASY routine for us. At that age, getting grumpy after 1-1.5hrs awake time, sounds like he's probably not hungry. The waking after 40 mins could mean that he is overtired.
Some babies never get to 'waiting' every 4hours. (mine never did)
Can I suggest that you direct some questions over to the Breastfeeding board and see if that can help first.
your milk won't have decreased over the past 6 weeks, around that time when age 4months, breasts milk supply is more 'regulated', which is entirely NORMAL.
I'd try fiddling with nap times / feeding time etc first before giving up on the bf, find that you have the same problems with ff as well.
In terms of ff - there are roughly the same number of calories in bf and ff, so may not actually affect weight gain.
Being sick is naturally going to have affected him and it will take time for him to put on good weight - he is using up calories etc on getting better.
See you over on the breastfeeding board.
alex