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.
The str 4 Cold One knights dates from either the 6th edition revision for Dark Elves or 7th edition, along with their ld boost to 9, in order to encourage their use in lists. Also weren't Blood Knights nerfed to str 4 in the legacy/legend list?
Blood knights are now strength 4 down from 5 in 8th. Might be just a limit on what non monsterous things can do.