Here's how I'd look at this.
If it's a 12 team league, each week, your league will be starting 24 RBs. There are only 32 starting RB's in the NFL, and some of them are in comitteee's, so the true count is even lower.
For WRs, your league will be starting 48 WRs each week. There are somewhere around 75 to 80 starting WRs in the NFL, given the variety of sets that teams use, anywhere from 2WR to 4WR formations.
So, your league will be utilizing almost 75% of the available starting RB talent in lineups and only around 60% to 65% of the available starting WR talent.
Has your commish balanced the value of WR and RB with 4 starting WR spots? No, but you are closer than in some leagues.
The fact remains that you should still find the true RB talent dissappear faster than you will the WR talent in this format.
As for an auction allocation, I would not pre-allocate equal $'s between RBs and WRs in this format. I'd still be willing to spend a higher proportion of my $'s on RBs.
What I would do is spend extra time studying who the possible breakout and high upsdie WR guys are that might be available in Rounds 11 and beyond in a traditional snake draft. I'd also reserve maybe an extra roster spot or two for WRs and grab a few of these high upside guys on the cheap in your auction for a buck or two. Could pay big benefits.
It may actually benefit you to have the other owners THINK that RBs and WRs are equally valued based on the 4WR starting requirement.