Tyres.co.uk has options for Michelin Pilot Sport 5 and Pirelli P Zero (PZ4) 255/45 R19 (104Y) XL for the W223 S 500 4MATIC (19-inch wheels). Please check the fuel-filler-flap label and registration. Other S-Class variants and wheel configurations may have different fitments.
The tyre size can be found in the fuel-filler flap on the approved combinations of wheels and tyres, as well as the tyre pressure table for the Mercedes S-Class. While the size of the tyre is found on the sidewall of the tyre, the specification for the particular model and wheels, as well as the load index, is found in the owner's manual and the registration.
The Mercedes S-Class may not need four new tyres, but it is recommended that two new tyres be placed on the same axle. It is recommended by UK tyre safety, that if two tyres need replacing, they be fitted to the rear axle. Mercedes S-Class 4MATIC versions should keep the same tyres across all 4 wheels.