Alot of things to consider when going Ko4 over the KO3's
ATB diff or you will be spinning the wheels and not puting the power down. KO3,s spools up faster than a KO4. useable power is what you want. its no good being the first to replace your tyres every other month.
A good KO3's setup can still get 230+ to 240 BHP, remains punchy round town and feels responsive. KO4 great for motorway's ect and top speeds 70 to 100mph + depends how long you want to keep your license.
when i did mine i could have gone the KO4 route but after talking to people in the know and wanting to keep the car drivable round town ect i opted for the KO3's, decat, 4bar FPR, DSG intake, Forge TIP, FMIC, pipercross panel filter, miltek resonated. uprated brakes 312mm, N75 J valve, forge DV, G60 flywheel VR6 clutch, R32 dogbone mount, or for more power powerflx bush's, but expect vibration. and an R-tech stage 2 map. and on the liquid gauge it has shown 238bhp suposed to be dam close to actual.
things to be carefull of going KO4
GTi driveshafts are not that strong, spin the wheels to much on a grippy surface like shell grip that you get at roundabouts, junctions ect, the wheels bounce and the shock you put through the drivetrain is massive. I have already broken one and at £200 plus can become an expensive item. you can modify Beatle RSI drive shafts, but thats real expensive and they can still fail. the more power the more weak spots you will find through out the drive chain ect.
another thing to consider is engine life, in the AGU lump the internals are very strong, very simular to the BAM engine. later engines used fracture con rods. not as strong! so if you go KO4 you might well have to upgrade internals aswell. diffrent injectors, the list goes on. yes you will have lots of power but can you put it down on the road and how long will everything else last. do you want engine rebuilds added to the list.
basicly it depends how deep your pockets are, and how much you can keep puting into the car to keep it tip top.
just some things to consider, so think hard and see how much your willing to spend and keep spending till she goes POP!