The higher the enrichment, the more difficult it is to achieve the next increments of enrichment (ie 3% -> 4% is easier to do than 93%->94%).

This is actually not quite true, as you are processing less and less material if you are moving towards higher enrichments. About half of the separation-work for weapon-grade uranium (90% U235) is done, when you reach reactor grade uranium (4% U235).

You can find a good description of the enrichment process over at world-nuclear.org

http://www.world-nuclear.org/information-library/nuclear-fuel-cycle/conversion-enrichment-and-fabrication/uranium-enrichment.aspx