Got a side grinder that uses 4.5" cuttoff wheels.

The one on there now is worn way down. I've got a pack of new ones. The
problem is that I don't clearly see the way to put a new one on. I know it
has to be fairly simple, but there are several possible things for me to try
to turn to get the wheel loose.

There is a gizmo in the front center of the wheel with four small holes
spaced around it. Looks like something that would take a quad spanner if
this is to be turned.

I do have a button on the grinder that locks the shaft, so that's good.
Between the wheel and grinder body there is an approx .5 dia shaft with two
opposed flat spots that looks like maybe it's for a real thin open end
wrench.

I don't have an open end wrench that will go between the grinder body and
wheel, nor do I have a 4 prong spanner.

Before I go to Home Depot and Lowes on the hunt for something, I need to
know what I'm going for. Is it a 4 prong spanner? Or a thin open end
wrench I need? Or what?