If the sphere is expanding, then the result is the same -- a gradual decline in energy density per unit of volume. So the specific geometry of the universe is unrelated to Olbers' Paradox.
But there is strong evidence that the universe is geometrically flat at large scales, which in turn argues that it's infinite in size.
How does the apparent large-scale flatness of the universe make an argument for an infinite size? To explain, and just as a simplifying example, imagine that the universe is the surface of sphere. Now include the implications of the fact that we observe large-scale flatness.
Picture this -- imagine that the universe is the surface of a sphere, but the sphere's surface is perfectly flat. How large must the sphere's radius be for its surface to be perfectly flat? Think about how a sphere's surface is defined -- it's the unique surface that's equidistant from the center of the sphere, the surface that has a distance of R (R = radius).
To make both properties true -- to accommodate (a) that it is a sphere and (b) that its surface is perfectly flat, all you need to do is make the radius infinite.
In the case of the universe, with more dimensions, to achieve the measured large-scale flatness, all one need do is assume that the universe is infinite in size.
> if the speed is greater or equal to speed of light, how we can be sure that we won't see ourselves in telescope?
That doesn't follow. We can't see an object moving away at greater than C, so we also can't see ourselves.
There is one place where we might see the backs of our own heads -- while observing at the event horizon of a black hole, all practical considerations aside. At that location, the spacetime curvature is such that light emitted "horizontally" would curve around the horizon (and the black hole) and reappear 360 degree away -- for example, from the opposite direction of someone shining a laser beam "horizontally" along the horizon.
> I mean, even if the radius is infinite, because speed is increasing, we should see sun light returning to us over universe surface at some point
> not that long ago we didn't know that we live on a sphere
Actually, the ancient Greeks knew it was a sphere, and everyone knew by the Middle Ages. Surprisingly perhaps, given their other astronomical work, the Chinese were the last hangers on. The notion that we discovered it relatively recently seems to have been due to a propaganda campaign in the 19th Century.
what if universe is a 4 dimensional sphere?