Thanks for writing in. Traditionally most schools only look at your highest GMAT score, so on principle a 4th attempt should not hurt your chances. Thinking more broadly though, is there a reason to think that this 4th attempt will significantly boost your score -- perhaps you weren't feeling well in the previous attempt or there was an issue at the testing center?At a certain point you may realize that you have maxed out your score and the prep time would pay off more if you focused on your essays/application -- after all GMAT is just one component of the admissions process.
Yes a 590 is going to make it very difficult to get into LBS or a top-10 school. Given this, if you think you have a chance to significantly improve your score with a 4th chance, there is certainly no harm in taking it again.