Then stop, and write about Source B, comparing back as you go.
This is easier mentally, under exam pressure. It is easier to find comparisons. But, I tried this method when I sat the exam last year, and I found that I ended up writing too much on Source A and not enough on Source B.
So the other method: write about Source A and then compare immediately to Source B, might be better for some of you.
The answer isn’t so much which method works best, it is which method have you practised? Practice is the key to both question 4s and both question 5s, because they carry the most marks.
To be honest, I would make this 80% of your English language revision. I wouldn’t bother with questions 2 and 3 until April.