This is confusing, but it seems to me both are official:
https://visas-immigration.service.gov.uk/is the original one, and
https://www.visa4uk.fco.gov.uk/is new, according to the explanation on the link:
"This is the new UK visa application website. We are testing it. It is designed to make the application process simpler and clearer.
Welcome to the Visa4UK service, you can apply for a UK visa using this service if you’re an applicant who is not currently in the UK."
But it does say the new one is for applicant who is outside the UK.