So, anyone kind enough to pinpoint the differences between this and MH4U?
Ooof. There's a lot. Graphical fidelity is different, which is immediately obvious. Very rough and tongue in cheek summary: 4U focuses on Monsters and maps, while Generations focuses on flashy attack effects.
Generations introduces prowler mode and Hunter Arts & Styles, which seriously change the way you play the game.
Prowler mode slashes your defense and attack and restricts your weapon choice(/individuality) severely in exchange for unlimited everything - no weapon sharpness management, limitless access to a choice of healing, traps, flash bombs, barrel bombs, horns. There's no need for paintballs, since you have a Psychic-like skill working at all times, and unlimited tranq bombs are available as soon as a monster is inside a trap. Gather as long as you want, no need for pickaxes etc. Also, no more stamina management: Sprint all the time. Hide in the earth to become invulnerable. Eat two, possibly up to three deaths before getting carted, with on-the-spot revives.
Hunter Arts and Styles for the traditional Hunter mode on the other hand seriously impact gameplay. Adept gives you an outright ridiculous invul-dodge that also conveniently powers up your attacks. Aerial lets you jump off monsters (even some attacks) and extends your dodge distance. Guild lets you indulge in Arts, which offer various effects like AoE heals, furious attacks, stat boosts, transporter boosts, etc.
There's also a far enhanced viability for ranged attackers in Generations. Bowguns now come with free built-in ammo mags, which considerably cuts down on micromanagement. Certain arts and styles give you free, or at least hassle-free reloads (e.g. Jumping off a monster with Aerial reloads your current ammo).
SnS gets a major boost in Generations with oils, consumable items that add a time-limited extra effect to your attacks. Exhaust, no bounce, etc. pp.
4U forces you through a coherent story, Generations reverts to the traditional hands-off style. Strangely enough, I feel Generations is far more verbose about everything. So much useless dialogue and pointless chasing after speech bubbles. Related: If you care for multiplayer, 4U unlocks that only after you beat Seltas, while Generations offers everything from the get go.
Generations is far more generous with early access to essential items like armor spheres or trap tools, which can be gotten for points.
Speaking of which, in 4U points are the more scarce resource, in Generations it's Zenny. Trade for godbugs and armor spheres for money, and keep an eye out for sales on genprey fangs for a bit of extra cash.
There's also a different Palico minigame, and the cooking system has been tweaked.
And, Generations lets you freely change Palico enemy affinity over 4U where it was an armour skill, and it offers a limited amount of customisability. You can transfer one skill and one ability from other Palicos. Oh, Palicos now have a chunk of skills and abilities you can pick from based on a slot limit.
Finally, on quests: village mode in Generations is outright painful. Way too many gathering and mini monster slay quests. Quite restricted "boss monster" variety until later. Offset by a huge assortment of themed Jaggies, err I mean _____-dromes, and a total lack of challenge until later in the game.
There's loads more.