All of Marantz receivers are rated for 6 Ohm minimum. I'm running 4 Ohm speakers on my 8300 and it does get warmer than with my previous 8 Ohm speakers, though it will do it. The problem is, if you have all 4 Ohm speakers or pretty inefficient/current hungry ones, they will still stretch the limit of the 8400 (still better than the 7400 though).
At $500, you are pretty much going to have to look used, because there are almost no 4 Ohm capable receivers at that price. About the only one I know would be the NAD T743 which MSRPs about $700, but can be found for a little less than that online from an authorized dealer.