The main difference between four and five star is the level of service e.g. having a concierge.

If you join the DesignClub and access the Guide to Hotel Design you will find (for example) my view of the required standard for both four and five star hotels. The DesignClub also has the UK guidelines on hotel standards at various levels within the download area.

UK standards generally are lower than elsewhere.For example in Europe now most countries are in agreement that the bedroom area should have a separate toilet, a standard largely ignored by UK hoteliers.