As a rule, it's best not to round in intermediate steps at all.
Let your calculator do the work.
If you have to (because you're starting to lose track), you can round, but always keep a couple of extra digits when you do.
Apparently you are doing an exercise to train the latter option.
IMO, to get the best result, you should still round as little as possible.
But it seems you are supposed to round a couple of intermediate results to 4 digits.
You can pick yourself what you want to round.
Btw, your formula appears to contain a typo.
I think it should be: x1=(-2a)/(b+sqrt(b^2-4ac)).