Hi, I'm currently moving over to cloth (building up stash when I see a bargain, when I can afford it etc) and I'll admit it was mind boggling the different choices out there. Firstly you need to check if you live in a local authority that does a cloth nappy initiative - I.e. they'll give you money (credit to spend in/online nappy shops like Bambino Mio, etc) towards buying cloth nappies (I think it's about £30-£40 in some council boroughs although I don't live in one sadly) and or they have trial packs you get free or to lend so you can see which brands you prefer. I'll get you the link in a minute.
In regards to what you should look for, in my opinion they're all much of a muchness basically you can get an:
all-in-one (aio) nappy that has a waterproof cover with a sewn in nappy pad that you pull out to wash (this helps it dry quicker as bamboo/hemp take a while to dry) Tots bots EasyFit and Bambino Mio Miosolo are like this.
Or a two part nappy thingy (technical
) which consists of a separate waterproof cover with either poppers to attach a nappy pad or folds to tuck in a nappy pad or you can use flat nappy sheets to fold up into a nappy and attach using those three armed things called nappy nippas and then to help keep it on the baby/help stop leaks you put the cover on your baby after. Tot bots Peanut wraps and Bambino Mio Miosoft are examples of this - there ar obv other brands but they are what spring to mind.
With both you can use either disposable paper type liner or microfleece liner to help keep baby dry.
I'm buying a few of each to see how I go in regards to what fits better, lasts longer, doesn't leak/cause rashes and what I find washes easiest.
Also Bamboo is more absorbent than cotton or microfleece but takes longer to dry (hemp is more absorbent but not as mainstream) so ideally you want to have something that's a combination of both, so for heavy wetters/night time a bamboo pad on the bottom, a cotton/microfleece in the middle (to help wick away wet from baby and into the bamboo layer while help keeping baby dry) with a liner on the top - then your baby's bum on top of that 
Seeing as your baby is 4 month and assuming he's average weight/size he's probably going to be a size 2 (most mainstream cloth brands have size 1 as newborn/3 monthish) or if it's a 9lb to 35lb nappy then no need to worry.
I strongly recommend having a look at The Nappy Lady website as she has a lot of the main brands and an honest description of fit and strengths/weaknesses or each brand nappy. From what I've read the premium brands all get good reviews so there's no duff ones to watch out for that I've noticed and I've looked at a lot!
Sorry that was reeeally long! Will be back with links... 