You can replace D4 with D6, but not like this. Just reroll all D6 that are 5 or 6. Your options give the advantage to D6 in the 1 and 4 option and to the D4 in the 2 and 3 option. No equal outcomes even if you factor in the average.
Throw 1 D4 please. Ok I got 5.
How is a 10% worse roll in case of 2D4 not changing the balance?