Using OpenGL 3.3 or 4.x

I’m still modelling but I am wondering whether I should use OpenGL 3.3 or OpenGL 4.x. I am looking at writing a program that’s compatible with a lot of computers (c++) but if most people have OpenGL 4.x compliant graphics cards then I don’t think there is much use in using OpenGL 3.3

Ubuntu 64-bit, AMD Radeon HD 7700.

You can estimate the availability of GL4 hardware from the Steam hardware survey (Steam Hardware & Software Survey).
Note that MacOS X only supports GL 3.2 even on newer hardware.
Whether you should use GL 3 or 4 depends on the requirements of your app (do you need tessellation and compute shaders?).
If compatibility is a priority, target 3.2 or even 3.1 (HD3000 on win & linux afaik), this will get you 95% of all users (according to the survey noted above).

Thank you for the article. When I get to using OpenGL I will try to target it against 3.x