I shot with F4's for years, but I suggest that you go with a used F5, I've had them since they came out.
Focus is MUCH faster, and always nails it, also you have 5 focussing areas. My F4's were in the shop at least 1 to2 times a year. The F5's, I've had since they were introduced and they have never been in the shop.
For what its worth.
David A