Confusion on Meliodas' clones' power level
They have a power level of 420 each because, when a clone is made, it has half the power of the original. But if multiple clones are made, then this power that was halved, is split into the multiple clones. Meaning that the clones receive power split up from the original's half and not from the original itself.

ie, 420 x 4 = 1680

It is detailed by Merlin in Chapter 112 of the manga: