As per
@McGinger's response, it depends on the sort of person you are. I'm more of a scientist/maths person, so I would find IT easier.
When I was doing literature, I found it more difficult than doing maths (which is known as one of the more difficult A Levels in general) or the sciences.
If you're more likely to do well in economics and maths (as you're implying), then you're likely going to find IT easier. Having said that, if you don't enjoy it, there's no point in doing it for A Level (as per
@CJS 123987's response). I would pick another analytical subject instead e.g. physics, further maths (recommended for top end unis for economics), chemistry, geology, stats,
If you are passionate about literature, and you are willing to do what it takes to get a high grade in it, then by all means go for it.
If you are able to do 4 A Levels, I suppose you can do 3 A Levels + literature, so by the end of Year 12 you can be a better judge to whether to drop Lit or not. If you're confident in doing Lit, you can do it alongside Maths + your other A Level (or economics). If not, you can do it alongside Maths + your other 2 A Levels (one might be economics should you wish).
Note: you don't need economics A Level to go into an economics degree.