Latour, Kristeva, Derrida, Baudrillard, etc

[CountArach]

The main application of their ideas is in history and literature AFAIK, as a counterpoint to grand narrative (Marxist) history.

This. Philosophy is mostly applicable to social science.

As for the problem with 2+2=4, the way I see it is that while we can all accept it discursively and there are obvious practical uses to it, there is no inherent meaning in the signified "2" nor the signified "+" and hence it is hard to defend their existence as a real and extant phenomenon. It is possible to at least conceive of a mathematical system built on a different numerical system which can explain the signified "4" without reference to "2" or "+" or which would not even seek to discover "4" but rather have a different formulation for that same signified. That is to say, "2+2=4" is only one way of expressing a problem (an explanation "=" of "4") and one discursively-grounded way of discovering it - "2+2". It is about breaking down our assumptions about what is inherent and instead showing that our ideas about reality are inescapably grounded in a socio-linguistic discourse.

I'm not sure what this all leads to, but how do we know that it doesn't lead anywhere without exploring it? Also note that I'm not defending the logic of the exploration, and this is only my guess at their argument, but rather I think that it is important that all disciplines are examined from a post-structuralist perspective because it can often push new things into the limelight that people had not considered before. It is happening right now with history and it will almost certainly happen to the sciences at some point.