The Raymarine ST4000+ drive belt is no longer sold by Raymarine. It may still be available elsewhere, and for a much lower cost than Raymarine's original price.
Go to Beltcorp.com , which now apparently is under Megadyne , part number "9.5AT5-750".
It appears to be the same underlying brand that Raymarine had carried.
I don't believe it's listed on their website.