If I am community college student, can I transfer to a more prestigious college like Pomona or Swarthmore after only 2 semesters? If so, is it common? What are my chances if I have 4.0?
They might consider you if your record is good. Usually they will say on their web site if they don’t consider sophomore transfers. Look up the Common Data Set for each, section D, to see what they use for transfer admissions.
At super-selective schools (10% or so xfer acceptance rate) the only thing that is common is rejection 
A school accepting a soph xfer only has 1 semester of grades to look at, so I’d expect them to look for other evidence you are a top student such as test scores or HS GPA & courses taken.
Most people complete two years at a CC and get their associates before transferring. Additionally, many schools tend to be more lax on high school grades and stats after an applicant as two years of college experience under their belt.