Using 4 ohm speakers with ANY receiver is a BAD CHOICE. You may get away with it if you don't listen very loudly. But if you have height speaker capability inside the AVR, the more 4 ohm speakers you install in the system, the higher the current load will be on the AVR amplifiers. AVR amplifiers are current-starved compared to outboard amplifiers that put 2 or more amplifier channels in a box of their own. A really good 2 channel amplifier will weigh more than a than an entire 11-channel AVR because the amplifier's power supply is GIGANTIC compared to the rather dainty power supplies in AVRs. That said, there are AVRs out there that can drive 4 ohm speakers EASILY... but those AVRs cost $10,000 and more. Your Japanese AVR is definitely in the current-limited dainty-power supply category. Honestly, I don't think you should risk using the 4 Ohm speakers with that AVR. SOMETHING in the AVR is going to get much hotter than it usually runs because of the low impedance of the 2 or 4 height speakers you want to use. The more speakers you are connecting to an AVR, the more you want them to be ABOVE 6 Ohms... meaning 8 Ohms. If you are going to connect seven 6-ohm speakers and four more 4-Ohm speakers, none of the speakers in your system will offer the AVR any "easing of the load" and it could be possible to engage over-temperature protection in the AVR that will shut-down the AVR if it gets too hot. If you are connecting 7 speakers rated by the manufacturer as being 8 Ohm speakers and you connect four 4-Ohm speakers to the height channels... you might get away with that. But the louder you listen the hotter the AVRwill get. I would suggest using as many 8-ohm speakers as possible, using NO 4 Ohm speakers at all, and using 6-Ohm speakers only in the more lightly used height channels. This way you won't be putting the AVR under undue stress by having too many low-impedance loads being driven.
Also be aware that when a manufacturer "rates" their speaker as 8-Ohms or 6-Ohms or 4-Ohms... those are just "guidelines". There is no such thing as a speaker with the same input impedance at every frequency. The crossovers and drivers interact to cause fairly significant variations in impedance at different fewquencies and phase angles. A speaker rated as an 8-ohm speaker is very likely to have an impedance of 4 or 5 ohms at some frequency ranges, 8-0hms at SOME frequency ranges, and 10-20 Ohm impedance at other frequencies. So "8" or "6" or "4" are just sort of "averages" of impedance. Speaers rated as 4 Ohms, will have some frequencies where the real impedance is in the 1.8 to 2 Ohm range and if that happens at a high-ish phase angle (from the crossover or driver(s)), those 4-Ohm speakers can be BRUTAL loads on an AVR. You MUST ASSUME that when a speaker has an impedance rating that there is at least one, and probably 3 or 4 or 6 operating ranges (of frequency) where the 4 Ohm speaker is 2 Ohms or less. That is asking too much of an a AVR... their power supplies are not robust enough to deal with impedance dips that low and do it gracefully.
