Maybe it's just because I'm not familiar enough with 3E, but I don't really see how 4e is more structured in its mechanics than 3E is.My take on this issue is 4E tried to compete with WoW by emphasizing similarities and the more structured mechanics in MMOs.
5E is trying to compete with WoW by emphasizing the differences, the human factor of the DM.
1st ed AD&D also had rather structured mechanics, at least as written.
You use the word "emphasise", and maybe you are intending that to carry a lot of the weight of your comment: ie it's not so much that 4e is atypically structured in its mechanics, but that it emphasises that structure.
I can think of two different ways of emphasising a mechanical feature (there are probably others). One is this: the play experience makes the feature salient to the participants in the game. Here are two examples where I think 4e does this more than 1st ed AD&D: in-combat movement rules; and hit points/damage.
AD&D has rules for in-combat movement speed. However, it's rules around positioning and AoE mean that movement speed is mostly an issue when closing into, or retreating from, melee, but don't really come into play when a melee itself is being resolved. Whereas in 4e, with its rules for OAs, forced movement, etc makes the movement rules salient not only at the beginning and end of a melee, but during it also. So 4e emphasises the structured movement rules in a way that AD&D doesn't.
AD&D, like 4e, treats a lot of hit point damage to PCs as non-physical. But it doesn't generally make this salient in play; whereas 4e, with second wind and inspirational healing tied to healing surges, does so. So 4e emphasises the non-physical character of hit points in a way that AD&D doesn't. (This isn't really about structured mechanics, but it still illustrates a way in which a system can emphasise a certain feature of the mechanics.)
The other way in which a RPG system can emphasise a mechanical feature is via presentation. To me, the functional difference between a 4e wizard's spell abilities and an AD&D wizard's spell abilities is relatively modest: both choose abilities from a list, with some choices being made during creation and others after a long rest; and both have a certain rationing scheme for those abilities. The differences pertain to the length of the list, the scope of the choices to be made after a long rest, and the details of the rationing scheme. At high levels, these differences are likely to be highly noticeable but at the lowest levels of play I think they're really rather small (the existence, in 4e, of at-will spells and short-rest recovery spells is the most noticeable one).
But some people seem to think the presentation of a 4e wizard's spell abilities, as instances of a more general shceme of AEDU powers, makes them radically different from the more traditional presentation in the form of a spell-slot per level chart and an appendix of spells filling the back half of the book.
For these people, at least, the 4e presentation emphasises the structure of wizard magic (in terms of choosing from lists, rationing at certain rates, etc) in a way the traditional presentation doesn't, even though in terms of play experience the difference is really quite modest, and mostly pertains to the details of rationing.