I found it on wikipedia : "A Lawful Permanent Resident can apply for United States citizenship, or naturalization, after five years of residency. This period is shortened to three years if married to a U.S. citizen, or four years if permanent residency was received through asylum."
Is that true that a permanent resident can apply for citizenship after 4 years if got his residency through asylum?

LPR can apply for natz at 4 years, 9 months (unless married to USC). Wiki needs to be corrected. They may be confusing the application for PA within one year of entry, but the time required as an LPR to qualify for natz starts at LPR approval date (even conditional PR).