Emphasis on the "rank." No doubt he thinks that for a man of his position in society, 4000 a year is hardly enough. Things we would consider luxuries, he is convinced are necessities (and if he were not convinced, Fanny would convince him).

See, this is one of the reasons why at times I despise John even more than Fanny. Fanny's an evil harpy, to be sure, but she is something, while John is merely a jellyfish. In Dante's Inferno you find him in that circle on the outskirts that consists of those people who would never commit to anything, good or evil, but followed the ever-shifting banner. It's no compliment to him to say that he would be better with a better wife---just another indicator of what an invertebrate he is.