Don't know about a RTX 4090, but my Radeon RX 6700XT has a much higher delta-T between GPU and hotspot running Furmark and being cooled by an Alphacool block, this without causing problems. So from my point of view your temps look just fine, no actually pretty good.
When stopping Furmark within seconds the Alphacool block cools the hotspot with closing in to the GPU temp, see below:
So to me it looks like a high hotspot temp is not necessarily a cooling block thing. Could also have to do with the TIM and application thereof and/or heat distribution in the die. So as far as I know it can differ from die to die just like the performance (
silicon lottery).
The lines in your graph run pretty much parallel so your cooling solution seems to be fine.
But that's just me with my logic

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