After longish downhills my hope m4 brakes tend to pump up making the lever quite hard. Is this a sign that my brakes need to be bled?

It could be a sign that it needs a bleed or it could be a sign that you’re dragging your brakes all the time rather than when you need to correct your speed.

excuse my ignorance but is it not normally a sign that the fluid needs to be changed?

Nope, if the fluid needed changing it’d probably boil and allow the lever to come back to the bars.Just a symptom of the fluid expanding with heat, but not enough room to allow it.