Cristian,

You may want to read this post for info on how to share pictures and or files.

As to your rendering problems:
- The forum has 18 posts/threads on "white spots" and 10 on "black spots". They may offer some insight in the problem.
- Q1 and Q2 rendering depends on hardware, and your video card may influence results. Q3 and Q4 rendering depends on software and should produce identical results, even between different computers.
- Ubuntu is fine, I'm using it myself with no more (or less) problems than people using another OS.
- Q3 uses a different rendering routine than Q4, so apart from a difference in the quality, there may be difference in the general appearance. Hence black spots in Q4 and not in Q3.
- You may want to experiment with the settings of the afflicted (furniture) models. Changing between Unchanged, Matt and Shiny may (or may not) solve the problem.
- You may want to use Q3 (which doesn't have the spots) and make an extra large rendering, which you then reduce in size to obtain the same quality as a Q4 rendering.
- You could share your design, so we could have a look at it and see if we can reproduce, isolate, or solve the problem, and/or offer a workaround.

Hans