I havent posted in these forums in a long time. But would like to try and help you out.
I'll start with my experience feeding babies pre-6months:
I started my first child (9 years ago) who was a premmie on solids at 4 months and she developed eczema for the first 6 years of her life and also had asthma until she was 3, was recurringly sick with colds and bronchitis. I dont know if this was because of solids or formula (I only breastfed for 3 months) or neither - just a bit on my observations. She is now 9 as I said and is a completely healthy girl, intelligent, fun, happy, very rarely sick, no eczema and no asthma and you wouldnt ever tell she was born prem.
My wee boy is 11 months old I started him on solids at 5.5mths with finger foods straight away (BLW) He was 10kgs at 9 months and that is big compared to other babies his age - he was near 5kgs at birth though (10lb8oz). He has also developed eczema and gluten and egg allergies! - probably introduced toast and eggs too early! But nontheless he still has a big apetite, eating 5 times a day and having 3 breast feeds during the day and 1 breast feed at night. He has had one cold over winter and thats it. Breastmilk is the goods!
My plunket nurse told me it was fine to start my boy at 4 months, but I decided to hold out.
Your breastmilk is completely fine, your breastmilk would be completely fine until your baby was 1 if thats what you wanted to do!
Everyone is told different by all different health professionals, and some people say, do it this way because thats what I did and it worked out, but all babies are different and all parenting styles are different, I can just offer up what I did and the outcomes of that, and also my beliefs which you dont have to listen to at all so I'll seperate it here for that - feel free not to read on - completely opinion based ->
I believe babies should really start solids at 6 months (though I didnt do that for either of mine) and I think you should keep milk as a first food until at least 9 months.
I do believe in being able to tell yourself what your baby needs.
Your body will make as much milk as your baby needs.
Keeping track of babys weight is a great idea at this stage, I'd check every 2 weeks and if his weight is going down then up the breastfeeds - not the solids.
Demand feeding is good in my opinion (though its not for everyone) you just have to learn your baby's cues. But if you feed your baby when he wants it then your body will make just the right amount so your baby will have it on tap whenever he wants it.
If you do use solids for a 4 month old - watties has a range specifically made for 4 month olds - here in NZ they are the blue labelled jars/cans - Im not experienced on making solids for a baby so young, but my first child was on store bought food until finger foods.
Dont feed your baby at this age based on "puts hand to mouth," "watches me when I eat" "sits up" etc. Those cues are for babies over 6 months.
If your baby is still hungry after a breast feed then feed him again, if he's having a growth spurt he may want to stay on the boob all day - and you need to cater to that, get a book, put your feet up, hire 10 dvds if need be, but babies do do this.
If you feed your baby and your boobs feel like they've nothing left to give, don't give up, baby will keep going and he will be getting the fat milk out just really slowly, but it does mean that tomorrow or the day after your boobs will be fuller and will accommodate his big appetite (speaking from experience here too).
At 4 months I would only be feeding once a day and then at 6 months may be up it to 2, my wee man wasnt on 3 meals a day until 7 months and then 5 solid feeds a day at 9 months and I started him early than most.
Well thats all I can offer - though it was a pretty long post sorry. Being a mum and trying to do whats right can be so hard, especially when no one can give you a straight answer on what right actually is. But empower yourself, do some hard reasearch, websites, call breastfeeding advisors, speak to more than one person such as gp or plunket nurse etc, read books, forums are great but can be confusing, find what feels right in your heart. Dont underestimate the power of breastmilk, it is the natural way to feed a baby it will definitely give your baby a complete feed, but when your baby wants it, not when you think its time.
I hope you get this figured out - good luck - would love to know how you get on. Here is a great site I still read with each stage of my breastfeeding I hope it helps to empower you a bit more.