I have attempted GMAT once and faced exactly the same problem.
. Since then I have been looking for ways to counter this problem. A couple of things that could really help in this situation:
1. Before giving the GMAT ensure that you have taken sufficient full length tests with exactly the same time breaks. This practice should habituate you with the real GMAT experience of 4Hrs at a stretch.
2. Use the 5 min breaks to relax, flex out, breath deeply, press your eyes gently to release the strain and splash some cold refreshing water on your face.
3. Meditiate for atleast 10 mins everyday to build your focus and concentration.(This really helps)