This is a grafted plant, the top a gymnocalycium that is red, cannot make its own energy, it uses the plant below to get energy. The plant below is a jungle cactus, and it has a short lifespan. When it dies, so does the upper plant. Most consider that the jungle cactus is not the best choice, but the industry has spoken.

It does not look to bad to me, it just has some years to it. Look out for rot. 4 years is a good length of time for these plants.

The lighter colored skin in the yellow oval is typical. It doesn't look like a problem. It seems to be a reaction to the cactus skin being close to the moist soil mix. I think that's just corking or something like that and that is a common issue with cacti as they age. It doesn't look nice but doesn't really harm the plant. If the skin becomes a different color, is softer, and looks wet inside then that is rot and you should worry about it.

The area in the orange circle is a bit concerning. It doesn't look rotten but it does look like maybe you let the rootstock go a bit too long without water so the rootstock has died back a bit at that top edge. Keep an eye on it. Mainly I'm concerned about it because it is close to the graft between the two plants.

I'm not sure what happened to the edges of the rootstock but it looks like corking- the same as the base.

The picture is out of focus but so far the top red Gymnoclycium looks OK.