@WandaVision Because it seems possible that your mortgage will be sold to a vulture fund, you should definitely re-fix. Do so ASAP. Even if your mortgage is sold to a vulture fund, you will at least have 4 or 7 years with the security of a fixed rate.
As for whether to fix for 4 or 7 years, here is how the repayments would look (approximately):
That extra €62 per month buys you an extra 3 years' "insurance" on a fixed rate. But your remaining balance will be about €1,500 higher after 4 years if you go for the 7-year rate (versus if you had gone for the 4-year fixed rate).
It is very difficult to say what fixed rates will be available from other lenders in 4 to 7 years' time, but at least 4% for a 5-year fixed rate seems plausible.
It should be possible for you to switch away from the vulture fund/the lender who is about to take over your mortgage in about 4 years (once 5 years have passed since the last item noted on your credit record) – but there are no guarantees, which might make the 7-year fix more appealing.
If you fix now for 7 years you might face a break fee if you try to switch after 4 years (or you might not, or it might be low). But even if the break fee is high in 4 years, you could wait and instead switch in about 6 years, at which point it will probably be a lot lower.
If the extra €62 per month feels worth it to you an extra 3 years' "insurance" on a fixed rate, fix for 7 years. But whatever you do, re-fix for either 4 or 7 years ASAP.