Why do some places say 4th generation hiv combo testing 3 months post pep is conclusive and others say only 6 weeks post pep? what are the odds of my 4 weeks post pep negative combo test result changing later?

No data exist on time to conclusive HIV testing after PEP. In absence of data, various experts have different judgments. Most agree 6 weeks is sufficient. Even those who recommend 3 months often believe 6 weeks is OK, but without clear data, they like providing a margin of safety in their advice. But most have never had a patient negative at 6 weeks with later turned out to be infected.