Using typographical variants of the same letter as mathematical symbols in a paper
Just an opinion: I think four variants of the same letter in one section is probably too much. In particular, I think (non-bolded) upright letters don’t work well for typesetting mathematics.

Also you say ”for four different variables”; this suggests that maybe you want to denote four of the same kind of thing in four different variants. (If I'm wrong, ignore the rest.) That is definitely to be avoided. As @Wrzlprmft says, the variant should reflect the kind of object you’re talking about, and related objects of different kinds should be denoted by the different variants of the same letter. The first example that comes to mind is a Lie group G and its Lie algebra  (blackletter g). 

Here’s an apposite quote from Littlewood’s Miscellany, via Milne: 


  It is said of Jordan's writings that if he had 4 things on the same footing (as a, b, c, d ) they would appear as a, M3', ε2, ∏"1,2.


Bad Jordan!