Need new rear tire

[Snoman6115]

Ive got a repair manual for my 79' xs400f and in the back it says 3.50s18-4pr but i get nothing when i search that for a tire size. i get the 18 means rim size im guessing 3.5 is rim width what is the "s" and the 4pr for? and the in the standard tire size format 00/00/r00 what size would i need? thanks

 

[nico]

http://www.webbikeworld.com/Motorcycle-tires/tire-data.htm

This came in handy when ordering my tires