I like the Nikor 4x5" tank, but some people I know have been frustrated with it. I think the key is to be sure the reel is properly adjusted and straight and to be sure to use enough solution to avoid uneven development (around 1100-1200 ml). Be sure it has the metal band that goes around the reel (though some people use a rubber band if they are missing the band).

I have successfully developed up to 12 sheets at a time in the Nikor tank.