Since you've already put in some study, four weeks is a decent amount of time to prepare for the GMAT - with dedication, determination, and lots and lots of practice, anything is achievable! Unfortunately, there are no "fast and easy" ways to prepare for this exam, unless you already have a natural aptitude for it (in which case I'm assuming you would not have made this post); it just takes time and practice.

It would help if you gave more specifics about your problems - what were your score breakdowns on your previous tests, what score you're hoping to obtain, what specific sections for quant you're having trouble in (even data sufficiency can be broken down into geometry, probability, etc.), what specific verbal sections you need help in, the list goes on and on. I've had quite a few non-native English speakers who've gotten great scores on the GMAT, so it is definitely not impossible!

Good luck!