To hit ssj4 with goku/vegeta follow these simple instructions!
1. you need a minimum of 12,000,000 (12 million) power level in base form.
2. damage yourself below 20hp (might be 10 or 15, but I think its 20 ;-) )
3. Press the ascend key (Default is Z) and then goku/vegeta will begin a different transformation process. Goku gets a sequence with emperor Pilaf wishing him back to childhood like in GT. Vegeta turns into a spinning clock to represent time passing and finally becomes future/GT vegeta.
4. See A and B below.
A. Collect the dragon balls and wish for "moon". As long as you still have the 12,000,000 power level you will be able to transform to SSJ4. Vegeta's tranformation is a single transformation whereas with Goku you must transform twice. The first transformation will be dirrectly to Super Saiyan 3 and then you can ascend again to hit super saiyan 4.
B. Raise your power level at base form to a whooping 30,000,000 and you will no longer need to wish for the moon before transforming. Both transformations will become instant although Goku will still have to do the super saiyan 3 transformation first.
Have fun and play fair!