Hi Daniel, if you are having ongoing feelings of instability and pain and you experienced a first time dislocation 4 years ago when you were 21 or 22 years of age then it is highly likely that you may have suffered a structural lesion i.e. a labral tear, where the soft tissue is torn of the bony socket. These are prevalent among Rugby players especially as a result of a tackle.
IMO no amount of physiotherapy or strengthening exercise is going to help stabilise a young structurally deficient shoulder. In a young patient like yourself surgery if appropriate is indicated and quite successful. Obviously your shoulder will need further investigation (MR Arthrogram) and a specialist examination.
I can empathise with you about not wanting surgery but it may be the only course of action. Once a shoulder has been affected by trauma then a compensatory muscle pattern develops which only makes the situation worse and you need specialist physiotherapy input to correct it to enhance your shoulders stability. This may have impacted on your previous efforts to strengthen your shoulder?In any case you will need this kind of therapy even after surgical intervention if you decide on that course of action.
I hope this helps with your decision making Daniel?
Cheers
Luke