Today I played this hand and I thought it can't hurt to look a bit into the math behind that. CO being in the hand varies a bit from the standard 'openraise -> 3bet -> 4bet' spot but here he is always gonna call so it actually makes this way more of a call. For this example tho I just ignore him being in the pot to figure out if I could call if it was HU only. 

EV(postflop) = Call% * (potsize after shove * avg EQ - flopshove)

OTF villain will shove 20,3 into 27,2 (discouting COs preflop-call) so we need 30% to call.
-> After we call pot will be = 13,6+13,6+20,3+20,3= 67,8
-> We have 30% on 38% of flops
-> I broke down the HvH graph (KsKh4s4h vs AA) in 4 segments and our avg. EQ when we call is ,70%

0,38 * (67,8 * ,70 - 20,3) = 10,32
-> The postflop EV for calling the 4bet is 10,32.

Since the 4bet is 10,20 more our call is pretty much breakeven (assuming villain autoshoves every flop and, as said, ignoring COs actions+contributions to potsize in this hand).

Is the math and logic behind this correct? 

Thanks in advance!