Yamaha RX-V4A: should the 4 Ω setting be switched on for Klipsch RP-600M II?
No. The speakers never fall below 4.4 Ω and the receiver is rated to 4 Ω. Switching it on «to be safe» trades away the 6.6 dB of headroom you have over reference level.
No. The speakers never go below 4.4 Ω, and the setting costs you headroom.
The RP-600M II is 8 Ω nominal with a 4.4 Ω minimum. The RX-V4A is rated to 4 Ω. The load never leaves the receiver's envelope, so the impedance selector addresses a problem you do not have. It is a protection mode, not a performance mode: it keeps the output stage inside its safe operating area with low-impedance loads, and it does not give the amplifier more current.
What it costs. At 2.5 m the receiver's 80 W into 8 Ω produce 111.6 dB — 6.6 dB above the 105 dB reference peak. That margin is what you would trade away for protection against a dip that does not exist.
While you have the spec sheet open. The 4 Ω rating of 115 W is measured at 0.9% THD; the 8 Ω rating of 80 W at 0.06%. The bigger number is not more power — it is the same amplifier measured at fifteen times the distortion. Denon publishes the same shape of curve: 70 W at 0.08% into 8 Ω, 110 W at 1.0% into 4 Ω. Whenever a receiver appears to gain power into a lower impedance, check the distortion column before believing it.
When you do want the setting on: a speaker whose published minimum sits at or below 4 Ω, or two pairs wired in parallel.