As I mentioned before, there is no problem doing what you are intending to do. I think people are trying to help but they may not know the SR18 has a very capable power supply and output devices. So to them, it is just an AVR that typically can't compare with a real "separate" power amps.
The SR18 AVR is not rated for 4 ohm, but given that it has a good size toroidal transformer, a total of 112,000 uf 71 V storage caps and output devices rated 17 A vs the typically 15 A rated for mid range to flag ship AVRs including the flag ship class SR8015, RX-A3080, and AVR-X8500H etc. I would have no concern using the SR18 with 4 ohm speakers as long as one doesn't go crazy with the volume.
I just found a review with some measurements that Marantz apparently provide to Gene:
Marantz SR-18 Follow-up | Audioholics
Based on that, I would say the SR18 should be at least as capable as the MM8807.