I see no reason to upgrade from the M4 Air to the M5 Air. I too have the 15" M4 Air. It is a wonderful computer and will last for many years to come.
On paper M5 will be faster than M4, SSD speeds will be better but on day to day usage, not going to notice any difference. In fact, upgrading from M3 or even M2, one is not likely to notice any difference.