I like the 4.20 purely as it equates to about 2.2 in the gf which it they beat Richmond they should be shorter than. For that reason and that reason alone you should back it.But then that makes your bet an all or nothing bet versus mine bet which is only risking 1 of my 2 units.
I'm only risking half my units in week 1 (so long as port are no greater than around $2.30 after the prelim thanks to cashout), whereas you are risking all your units in week 1
Cash outs are terrible value you are better off betting the 4.2 then backing Brisbane in the GF to hedge but then you are double dipping in their margins which gets rid of most the value of the 4.2.
Best bet would be 1U on port v tigers. 0.5 unit on port premiers. If you think port can go the whole way