Jo7249- I am sorry you are going through this- having a 4mo is worrying enough, without having this to deal with too.
I have had 2 dc with plagio unfortunately, and I believe that it is the child's disposition that will allow/not allow repositioning to work. 1 of mine had helmet treatment, the other was solved entirely by repositioning.
Please get you GP to check there isn't a neck problem (torticollis)- if there is they will give you some physio exercises to do with him.
Basically, I spent the first 6 mo of both their lives holding my babies upright- in a sling, a bumbo (ok from 4mo if they are strong enough), in my arms etc. (DD had reflux too, so was upright most of the time anyway)
DS was a v good sleeper, and was a baby that did not move in his sleep, not thrash around or anything- for him, we solved his plagio entirely by repositioning- using one of these at every nap and sleep. The first couple of days were tricky, but he soon got used to it- he still sleeps on his side now, on either side interchangeably. It took probably about 8-12 weeks to sort it out, and he has a beautifully round head now.
I cannot recommend the sleepcurve matresses at all- the sleepcurve we invested in (to prevent plagio in him, after our experience with dd) gave him plagio- I think possibly because he was a heavy baby, who never moved.
DD however was a demon in her sleep- could never lie still, terrible problems sleeping (maybe because of her reflux), and we had to have the cot at a steep angle too, so repositioning was a complete non-starter. We had the cranio band from Dr Blecher, and we were very pleased with the results. Yes (as someone upthread mentioned) they do have to wear it a lot - 23 hours a day... but there is no point in forking out 2k, and putting your child through the helmet wearing, unless you do it properly. Her measurements went from 32mm asymmetry to 4mm- an amazing improvement, over 4 and a half months.
I notice children with brachy- and plagio- cephaly all the time- simply because having a child with it makes you very sensitive to people's head shapes!
I hope you get some help from your GP. You can use rolled-up towels to help with tummy time, and sleep repositioning, though I found the bebecal thing excellent.
There are studies which compare treatment vs no treatment- some in the Netherlands, and some in Singapore(?? or Malaysia?) but it is 3 years since I researched it, so couldn't tell you the links now.
HTH