If I could actually do something with an iPad there would be a different discussion, but the operating system is so incredibly gimped that the most demanding task it's really suited for is .. decoding video.
Well, it's not. Every process shrink improves power efficiency. For watching videos, you're sipping power on the M4. For drawing...well if you want low latency while drawing, which generally speaking, people do, you...want the processor and display to ramp up to compensate and carry strokes as fast as possible?
Obviously if your main concern is the environment, you shouldn't upgrade and you should hold onto your existing model(s) until they die.
As for the environmental impact, whether I buy or don't buy this iPad (I won't, don't worry, my 2020 one still works), millions of people will. I don't mind people buying powerful machines when the software can make use of the performance, but for iPad OS..?
A faster SoC can finish the task with better "work done/watt". Thus, it's more environmentally friendly. Unless you're referring to the resources dedicated to advancing computers such as the food engineers eat and the electricity chip fabs require.
Regardless, the environmental impact I'm thinking about isn't mainly power consumption.
Millions of people will be buying these things regardless of what I'm doing.
So what are the performance cores doing there?
Apple's started to roll out green energy charging to devices: https://support.apple.com/en-us/108068
If I had to ballpark estimate this, your iPad probably uses less energy per year than a strand of incandescent holiday lights does in a week. Maybe somebody can work out that math.