How accurate is the 4th generation combo test at 6 weeks and 8 weeks?

100% at both times: The test is conclusive any time from 6 weeks after infection for the life of the tested person. The only known exception is if the exposed person was taking anti-HIV drugs around that time, i.e. pre- or post-exposure prophylaxis (PrEP, PEP). In that situation, many experts would recommend testing at 3 months and maybe even 6 months.