If anyone has experience with this, I'd love some help: what's the difference between applying for a UK visa through visa4k and through the main portal gov.uk? Can you tell me whether both are reliable?
Answering this question quite late I found that there is no difference.
The visa4k site is closed and there is a link that leads to the same site as the gov.uk link.
So while there used to be two ways, or even two different sits, it is one site only by now and that is the official UK governmental site. And that is a reliable site.