My 24" 2025 iMac with M4 chip running Sequoia 15.7.8 getting very hot

How hot is "hot?" Can you hear the fans ramp up? That is 100% normal and the designed respond to detected heat increases. If the fans cannot cool it enough, the processor clocks back to reduce its load. If that does not help, the computer does a safety shutdown to recover.

All devices get hotter as they use energy. Humans too. Heat production is directly related to the load placed on the computer's main processor and video processor.

So...what apps/activities are you running when you observe this heating?

Workstation arrangement can affect this as well, but is easy to check and, if needed, fix. Your cool air intakes and hot air exhaust functions are along the bottom edge of the case. That ~3.5-inches/90mm of clearance between the case edge and the work surface is careful designed to allow proper cooling. Place nothing between the case bottom and the work surface.

This is an older iMac but the intake vent location is the same as in your M4 iMac. Its owner posted this image here wondering how their iMac could be running so hot:

When I worked in law enforcement, we called that a "clue."

It is normal for games with complex graphics, streaming video, and video-conferencing to run up the temps as they work both the processor and the graphics hardware hard. That what the internal fans are for.

Software: The following aftermarket add-ons are totally unneeded and will run up your temps and degrade performance:

—anti-virus software.

—so-called "cleaning" or "optimization" apps

— VPNs not issued by an employer

Also Google Chrome is known to be resource-greedy.