Fina for 4 weeks is definately OK, although as everyone says, longer would be better. I gained close to 20 lbs in 3 weeks on 50 mg fina ed. I think I may be more an exception than the rule however.
I just got my bloodwork back after week 5 and all my liver and kidney values are virtually unchanged from my baseline taken before my cycle began. I think all that hype about fina being hard on the kidneys is just that, HYPE.
-Spidey