The minimum for medicine is five highers for Scottish universities with the exception of Aberdeen. They say you can apply with four if this school policy or if you have extenuating circumstances but you must provide written evidence that this is the case. Check out their website it's pretty clear.
But there are also some English unis which look at highers and advance highers combined so you would get away with four highers in fifth year.
You just need to do a little research.
Hope this helps.