Paladins dont use cleric spells they use spells from the paladin spell list, some of which overlap with the cleric spell list.
And since as you said paladins dont get very many spell slots or levels the rings would be overpowering.
Level 4 slots for paladins are the paladin equivalent (in terms of the level they get them at, not necissarily in terms of power) of level 8 or 9 cleric spells. If there were to be a paladin/ranger ring, there would only be a first level one, since since anything higher is the level equivalent of higher than 3rd level spells for wiz/sorc/cleric. (the spell ring series ends at third level spells for wiz/sorc/cleric.)