Do a bit of checking first and only look at that if there's an issue. What bar mixer is it, do you know the spec?
If the base of the Cold Water Storage Cistern (CWSC) is 4m above the level of where the shower head will be then you may be ok as that'll supply up to .4bar to the hot supply, pipe runs dependent. It'll come down to the difference between the mains pressure and the pressure of the hot feed.
Whatever the result, if there's an issue there will be a way to sort it out.