it could be an IBS issue. The increased excitement of being new to training, increased adrenaline, the body adapting........even possibly the glee of potentially dropping a dress size
. It can lead to symptoms that are more linked to stress related conditions, even though you dont actually feel stressed. Might you ever found a similar bloating or discomfort after a really bad few days at work or after a tough time in your personal life?
Dare I say you might doing a little too much for someone in their 4th week? 40mins a day for a 4 week newbie is admirable/keen but probably too much. You might be 'running' yourself down, please excuse the pun.
I'd Google some beginner training plans which will show you with some variety the best way to go forward and improve as a runner.
Most competing athletes won't like to admit that they can be plagued by all sorts of bowel related annoyances around races, including 'runners trots' (diarrhea before, sometimes inconveniently during and after distance races).
As for a sodium problem? I presume you are referring to the effects of losing too much of the body's salts, minerals and electrolytes during training. I can't say that I'm aware of there being a link between that and stomach bloating. However, make sure that you are well nurished and hydrated prior to excercise (obviously leave exercise at least 60 mins after eating) and make sure that you eat and drink as soon as possible (within 30 mins) of training to replace what the body has lost and to start building up glycogen again.