4e is built upon not having multiple magical defense boosters to the same defense in different slots. Adding them will screw up the maths and is a probably a bad idea.
Magic armour boosts your AC, magic neck-slot items boost your other defenses, and a few other factors (such as certain masterwork armours) will provide extra little boosts here and there to round things out. It's a delicate balance, and not one that should get anything extra layered on top of it.
Besides those? Plenty of things that grant conditional bonuses, but it sounds like you want a constant one. There are a few different items that grant a +1 bonus per tier to a specific defense - belt of vim, etc. (so the +3 Fort one is level 28). The other thing that comes to mind is the Defensive Staff.
Some belt grant a bonus to Fortitude, some boots to reflex and some head to will. The bonus is usually around +1 per tier. Apart from that, I can only think about the staff of defense as said above.
Apart from that, special material for armor can give bigger or different bonus (+For on plate for exemple).
Let me clarify: I'm asking about magic items that increase all four defenses (AC, Ref, For, Will) at the same time. MarkB's explanation helped me see why that might not be a good idea.
If ALL your players want this, it might be better to do some kind of rules fix - there has been much discussion about the brokeness of the 4E math here, specially NADs.
They do if they're a different sort of bonus, like "item" and untyped, or both untyped. If they are the same type, say both item bonuses, then you use the highest.