I'm not so opposed to a 4-team playoff in general terms as I am opposed to a 4-team playoff with 5 power conferences.
I'd probably still prefer 8, I just think it would be a lot more tidy if we didn't have fewer slots available than regularly "in the hunt" conferences. 4 still doesn't give the G5 a real shot but truly deserving G5 teams are quite few and far between; Even this year with a literally undefeated G5 team practically nobody argues that they have a chance to actually win the playoff, some just argue that they deserve a shot on principle.
There's always going to be an argument for expanding the playoff.
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Stopping at 8 leaves out deserving teams. Why not 16? Not much of a difference between the 16 and 17 teams, that doesn't seem fair. Why not 32?
There's always an argument to expand, yes, but the argument gets more flimsy the more teams you have. This is why there's no massive national uproar over the #1 seeds in the NIT being left out of the NCAA tournament.
Auburn was once an undefeated SEC champion that didn't get to play for the BCS title. No argument between #s 8 and 9 will ever come close to something like that.