Will using an M4 MacBook Pro on your laptop harm the laptop?

The real problem is ventilation, especially as the computer ages. Fabric is not a great heat conductor and can cause problems. Although some people may allow a gap between their thighs where this is not as bad a problem.

I also would not put the laptop on bare skin. People have been known to get burnt that way.

You can get nice "tray tables" that you can put on top of your lap at major office supply stores designed specifically for people who would need to use their machines on their laps a lot.

<< You can get nice "tray tables" >>

Great idea!

Some are marketed as "lap desks" -- basically, a bean-bag for your lap attached to the bottom of a nice smooth flat surface for your computer. Some have a small area for a rolly mouse.

Another TRICK for cooling by forming a convection current: you need about 3/4 inch (13 mm) clear airspace under the metal bottom to allow a convection current to form, and increase cooling. [I double the rubber feet under my Routers and Ethernet switches, and they stay MUCH cooler that way.]

I've been using my M1 & M2 MBPros on my laptop at times without any problems other than it is hard to keep the laptop in place due to the shape of the bottom case. I find trying to keep in place can sometime hurt my hands & wrist if used for longer periods of time. If I keep using my M1/M2 laptops on my lap, then I will need to invest in something, but I am hoping to get an old non-Apple laptop running again which didn't have a problem sitting comfortably on my lap.