Let X be the intersection point of AB,FH,CD, and let Y be the intersection point of BC,EG,AD. Let L1,L2 be the angle bisectors of ∠AXO,∠OXC respectively, and let K1,K2 be the angle bisectors of ∠AYO,∠OYC respectively.
Clearly, the centers of the inscribed circles of the four quadrilaterals (if they exist) must lie at the intersection points of Li,Kj (i,j=1,2). Let Pij denote the intersection point of Li and Kj. We set
α1=21∠AXO,α2=21∠OXC,β1=21∠AYO,β2=21∠OYC
The condition that the four quadrilaterals are tangential quadrilaterals is equivalent to the distance from Pij to one pair of opposite sides being equal to the distance to the other pair of opposite sides, which can be written as
XPijsinαi=YPijsinβj
Now let aij=XPijsinαi,bij=YPijsinβj, we want to prove that if aij=bij holds for three of (i,j)=(1,1),(1,2),(2,1),(2,2), then it also holds for the fourth.
Note that we have
XP12XP11⋅YP22YP12⋅XP21XP22⋅YP11YP21=SXYP12SXYP11⋅SXYP22SXYP12⋅SXYP21SXYP22⋅SXYP11SXYP21=1
1=a12a11⋅b22b12⋅a21a22⋅b11b21=b11a11⋅b22a22⋅a12b12⋅a21b21
Clearly, if three of the ratios on the right-hand side of the above equation are 1, then the fourth is also 1, which completes the proof!