You can do this, but you have to place the batt on the BEC side of the EVX and use a jumper to short the unused inputs.

I assume your y conn was intended to take a pr of 2S and place them in series for a 4S? Did you leave one side on the evx empty?

Use a 2S and attach it to one side leaving the other empty. Turn on EVX switch - if the light comes on, this is the BEC side of EVX - the 4S will conn here. The other side needs a simple jumper.