It depends on what you’ll be programming mostly. If you’re just looking to making some simple indicators/EAs via MT4, some C++ and/or MQL4 is all you need. MQL4 is derived from C++ so learning a bit of C++ won’t hurt.
But if you’re looking to do some more analysis with your own scripts and data sets, then perhaps something that allows rapid prototyping is better (Python, Ruby). Another alternative for data analysis is learning R or MATLAB.
If you’re interesting in building entire applications and such, .NET/C# would really come in handy as well. Not to mention this would be a good route to go if you wanted to create DLLs for your EAs.
Note: Try not to get overwhelmed! It seems like a lot at first; take things one step at a time.