@DanUK, what is your business purpose to upgrade your X-Cart? Mobile ready, HTTPS only mode, PHP7 Support, some important integrations?
So the first is the business goal, technical stuff is in the second queue.
You can easily try to migrate your data to X-Cart 5 using this addon:
https://market.x-cart.com/addons/migration-wizard.html
it is under the active beta testing and we are going to release it in public in 3-4 weeks.