61W USB-C power adapter usage?

You can use it to charge any and all of those devices. Your iPhone for example fast charges with 20W, but using a charger to supply more wattage will not make it charge any faster. 20W is the max. For your new computer, you can certainly also use the old charger, but it might just charge a bit slower. In terms of battery health, using a higher wattage charger will not hurt the product. The actual charger is in your devices and that regulates the power going into it.

Sure. There is a reason that the M4 MBP comes with a 70W power adapter. Using any adapter with a lower power rating may work, but battery recharging will take longer, and potentially, cannot charge properly over time.

So, go ahead and use it for your other devices, but use the 70W one for your laptop.

Short answer:

1. Yes
2. No

using a too small watt charger will not harm the device

when a device (laptop) draw more current than the charger can provide the charger will automatically lower the voltage (it's ohms law) and if lowered enough the device will stop accepting the connected charger as a charger.

back in the day when ipads came with 12watt ac chargers and iphones with a 5watt charger, then the ipads would charge very very slowly using the 5watt charger, so slow it would not show the ligthning bolt icon on the batt, but charge a bit all the same.

Chelley Vician1 wrote:

Your final point in your elaboration response ('cannot charge properly over time') would seem to argue for my point "it should not be used", but you had answered 'No' to my 2nd question, when perhaps you should have said "yes"

You are correct. After re-reading your post, my answer should have been "yes" for the 2nd question. Sorry about that.