I've been going in reverse order. I started with 6 and I just did form 3. I read in a few places that form 3 was tough but I found it to be the easiest out of 4, 5, and 6. Personally, I thought 6 was the hardest with 4 being a close runner-up.
I think if I had taken form 6 now rather than at the beginning, I would have found it to be easier. So honestly, I think it really depends on how far you are along with your studying and what subject areas you've been focusing on. It seemed that form 4 was heavy on biochemistry, which is one of my weak areas. If you know your biochem thoroughly, you may find form 4 to be easier.
My 2 cents.