Is your razor sharp? Has it been rehoned during the four months? You can still shave with a comparatively dull razor, but you will unconsciously use more pressure --> burn. Do you strop before every shave?
None of this is meant as criticism. But answers to these questions might lead us to a solution.
You should be able to get a passable shave with one N-S pass down the neck, and no way should this be irritating if everything else is in place.