Well, SSJ4 Goku's power level is an estimated 4 trillion cubed before he powers up. He has about 12,000 years of fighting experience, can shoot energy from his hands, has a tail and wears eye liner. The eye liner makes him approximately 7 times stronger, according to my extensive calculations.
On the other hand, the L-Block is perhaps the most evil shape to exist in tetris. How often have you accidentally misplaced the l-block, only to **** up an entire line? I'm sure the answer is quite often. I remember having lines with blocked off spaces. It upset me greatly. What if you were tired and put two l-blocks together, forming a square with spaces within? Does that not boil your blood?
Anyway, I think this battle would be a tie. The l-block would come flying at Goku who, after applying more eye liner, would swiftly turn around and "take it" as far as it could possibly go. Although it would probably severely weaken Goku, he would be by no means defeated. After all, he had been training with Vegeta for such an occasion over the last decade. The l-block simply can't die since it is a symbol of repression and hate, so it would remain a part of Goku for all time. At some point, Goku would teach Vegeta the art of receiving, and he would be free of the l-block. It would be Vegeta's burden to bear.
I think this is a fairly accurate description of the fight.
He had a lot of troubles defeating the cube, so i don't think he's up for the L - block. But maybe if he goes SSJ43058 then he might stand a chance, though by then he'd probably turn into cousin It or a kuriboh.