It is a high current design, so it should be able to handle your 4ohm speakers without stress.
Having said that, if you are running 7 speakers all nominally 4ohm but with a minimum of 1.6ohm (like my own Gallo speakers) - it might be pushing things! - But most 4ohm speakers don't dip down that far, and most configurations only have the "difficult" speakers as front LR or LCR... with "easier" speakers for surround and height duties.
I previously had two direct ancestors to the RX70 (Integra DTR70.4 and Onkyo SR876) - these both had well nigh identical specifications to the current RX70 - and they handled my difficult and inefficient speakers without issues.
My brand new Integra DRX3.4 (equivalent to Onkyo NR7100) - does NOT handle my speakers well, and I therefore run things with external power amps... - which I would not require if I was running an RZ70.
(I am waiting for the release of Dirac ART on Integra/Onkyo before hopefully upgrading and simplifying my setup ....ie: removing the external power amps!)
And P.S. there was no upgrade of chips on the Onkyo family.... it sounds like the AV shop guy was mixing up the Denon/Marantz family with the Onkyo/Integra family.
The Onkyo did have HDMI and DSP chipset issues in the AVR's released between circa 2008 and 2015 - but none of that applies to the current generation of equipment. (sadly it applied to the models that I owned

)