Certainly I'd call giving the Elf riders Strength 4 BS, as the vast majority of Elf units have Strength 3. I'm sure in previous editions of the game they also had Toughness 3 as they should, though this new edition makes the call less easy given that mount toughness is folded into rider toughness. Giving them Toughness 4 because of Cold Ones being tougher than the Elves themselves makes sense... but in the same vein one could consider that our Saurus Cavalry should indeed be Toughness 5 due to Saurus being tougher than Elves, plus benefitting from riding equally tough Cold Ones. I mean, Blood Knights have Strength 5, so it wouldn't be untoward to have a cavalry unit with Toughness 5 too.