You say the rear cylinders are not firing,
do you have spark,Are the valve clearences ok on those cylinders,
If it's reving on all four cylinders with choke then you have a pilot jet or circuit blockage.
It's time to get these carbs ultrasonically cleaned.
Otherwise you are pissing into the wind.

I double checked today, and as I said it sounded worse after cleaning the carbs, but I think I may have over reacted a bit, it just sounds different, so I sprayed water on the exhaust header pipes and it seems that one of the rear cylinders is working fine (water evaporated instantly) but then the water kind of lingered for a while on the other rear header. But now when spraying the front header pipes I experienced the same, one evaporating instantly and the other kind of just staying wet for about 30 seconds or so.

It also doesn't seem to make a difference whether the choke is on, all the cylinders still don't seem to fire at lower rpm.

I think I will give the valve clearances a go tomorrow and see what I find.

Thanks for advice and replies.

Okay so I checked the valve clearances, all the exhaust valves were out of tolerance at 0.29, inlet valves were all in tolerance... so from this I take it the slightly out of tolerance exhaust valves wouldn't be causing this huge problem? I think I will just pay a workshop for a quote tomorrow and see what they say is the issue with the bike.

the exhaust side is okay at 0.29 - the HRC set up manual has the exhaust gap at 0.3 (on the RVF) but i assume the vfr would be the same or very similar.

Well that's good to know, the bike went into the workshop today, will probably get a reply tomorrow with the quote, I hope they don't ask for crazy money.

Got the bike back today, all cylinders now firing 100% :D

New spark plugs were fitted, carbs were also given an ultrasonic cleaning. According to the mechanic the knocking/rattling noise which is still there at low idle is just a noisy gearbox and nothing to worry about. Fuel leak is also sorted, the carbs just needed new float bowl gaskets.

But when I got home I realized the bike won't idle and just dies if the choke isn't slightly open. So I attempted to adjust the idle screw only to find that they had broken the idle screw in half and the cable just shoved between the V, it also came back with no tank rubbers (the little blocks). I will have to take it back tomorrow for them to sort these two things out.

Thanks for all the help from everyone in trying to help me sort the issues out.