ONE. This question is very speculative, so I don't think a specific and technical answer is available. Scientists are extremely hesitant to call anything impossible (including both silicon-based life and a conclusive answer to this question). Certainly no one has yet proven that life can't be based on silicon, and because it has the same number of valence electrons in its p orbital (four), it is tempting to think that you could just swap out carbon for silicon.
Unfortunately, carbon-based chemical compounds tend to be just the right amount of "floppy." That is, large structures built on carbon like muscle tissues have enough cohesiveness and structure to maintain their architectural integrity when stressed, but enough flexibility to act as, well, muscles. Silicon-based structures, on the other hand, tend to fall on either side of the "Goldilocks" structure- either much too rigid (like silica sand), or much too gooey (silicone, as in breast implants). That is obviously a very broad answer, and I would welcome comments or edits on the subject.
TWO. This is an even longer extrapolation from known science, but we can probably guess. Someone with a stronger chemistry background than I might be able to assist. However, in discussing carbon-based life, we always talk about the critical importance of liquid water. I would imagine that a liquid solvent would be just as critical in the development of silicon-based life. Chemists, would water still be the gold standard for silicon solvents? Some other?
...So, for whatever solvent silicon life used, you'd need temperatures in its liquid range. Besides that, certainly an energy source. Light from a star, or chemical energy, or a heat gradient from the planet's warm core? Hydrogen, oxygen, and nitrogen will likely play the same roles for silicon life as carbon. They are the most abundant chemicals with their respective numbers of valence electrons.
THREE. Humans might be able to live peacefully side-by-side (or spacecraft-by-spacecraft) with silicon-based aliens, but there's a good chance that free carbon-based and silicon-based ecosystems on a single planet would be poisonous to each other. Even if not, they would have to compete for energy and available chemicals as mentioned in the previous question, so I think one would necessarily go extinct as it was out-competed.
->This is a really cool concept! Evolutionary biologists have examined competition and survival of the fittest at the genomic, organism, population, species levels, but survival of the fittest biochemistry? You might be able to make some predictions on the basis of efficiency of chlorophyll photosynthesis vs. the silicon energy source.
The only way I could see natural coexistence is if the two ecosystems were almost entirely non-interacting and non-competing. For instance, perhaps a land-, carbon-, and photosynthesis-based ecosystem coexisting with a deep sea or subterranean silicon- and chemical energy-based ecosystem.
FOUR. None whatsoever. Silicon-based life is purely speculative at this point.