LOL Tarmac, nice one

Krusty. There were to types of MG151's.
One called MG151/15 and one called MG151/20. The 15 was the 15mm heavy machine gun and the 20 was the 20mm cannon we have in AH.
Like Karnak said it is a myth (damn persistant one) that the 109 K4 used 15mm cowl guns. And the damn myth still lives on in some books together with the 30mm MK103 Hub cannon myth.
As for the FW 190 they never used 15mm guns, not in cowl or any others place.
As for the reason of changing the G10 to K4 name probarly because they wanna fill the gap in the plane set. Noone but HTC knows of any new 109's yet, maybe there won't be any. We're hoping for some late G modells though such as a G14 or a G6 AS. We'll see.
By adding the K4 they remove the G10/K4 hybrid. (There may very well have been some G10's with that engine aswell though).
As for it being perked in the MA I see no reason for that, the plane should perform like the G10 we have now but it won't have the 20mm hub cannon option.