I've had a little experience with 3,4 and 5k's. Honestly I wouldn't waste my time with a 4k. If you have to get another motor find a 5k.
Real world example - a cammed 5k took 2+ seconds a lap off my time around the Ballarat autocross track.
Real world example 2 - up a local hill, the 3k (std healthy engine) would struggle to hold 100km/h in 4th. With the 5k (std healthy), it would accelerate in 5th up the same hill.