I'm sorry that nobody has replied to your post sooner. I'm not sure what the official guidance on prescribing timescales is. My local surgery prefers to give me only 28 days at a time which I suspect helps their finances. I expect and hope that GPs have discretion to offer bigger batches if appropriate.
If you are physically able to go away for 4 months then it would seem unreasonable for your GP to restrict you in this way.
I hope you have managed to make some progress