Which weapon should I go with?

Starting with the basic attacks, your draw and your standard X attack are forward overhead slashes, these are your slowest moves, but also some of your strongest. When doing these, you move yourself forward a little bit, quite important for being able to space yourself and aid your approach. This move can be used twice in a row if you're using the first as a draw attack, otherwise it's just one use. It goes into the A attack, a jab, which is very fast but low damage, and you can sprinkle these in as much as you want, it may be just a little extra, but it adds up after a while. The third attack in the basic combo is an uppercut on the X button, a neat little upswing with a high vertical range. This attack isn't too special aside from being good at hitting high tails like on Rathian, Gravios, and the like, and can combo back into an overhead to repeat the combo. It's also worth noting that you can use the circle pad and adjust your angle with the jab and the uppercut, so you can better hit the monster instead of being awkwardly to the side a bit.

A very important feature of the LS is the fade slash. Press X+A together to do it. It has a wide horizontal range because it's a sweeping attack, but that's only noteworthy when clearing out annoying crap like Jaggi. The useful part is that it's a moderately strong attack that moves you a good distance, like an evade would. This is super good for repositioning yourself so that you can continue to attack a monster without letting up. However, while it's good for getting out of the way, the fade slash itself has no invincibility frames, so make sure you're out of the monster's hitbox entirely. You can use it after any pretty much any other move, so if a monster's about to attack, you can just interrupt your combo and use this. The default motion is directly backwards, but you can use the circle pad to move left or right as well.

The third main part of the longsword is by far the most important, and that's the Spirit Gauge. This builds up when landing attacks, and when you max out the bar, you'll flash red, and that'll prevent the bar from automatically decreasing and give you a small attack boost, all for about 30 seconds. However, you can use the Gauge even when it's not full, it's just handier to do so. By pressing R, you'll attack using the Spirit Gauge and do an attack called a Spirit Slash. They're quite high in damage relative to your other attacks, and are a combo in itself. The first one is a diagonal slash to the left, the second one is a diagonal slash to the right. The third one is actually three attacks in one, you only need to press R once for it, and you'll make two quick strikes before slashing down overhead. The fourth and most important is the Spirit Roundslash, a wide sweeping attack with a huge hitbox. Using this will automatically sheath your weapon in a cool animation, but landing a hit with it will also make your Spirit Gauge glow. The default is empty, the first level is white, the second level is yellow, and the third level is red. They give you an attack boost of 5% for white, 10% for yellow, and a whopping 30% for red. If you land a Spirit Roundslash while red, it'll just extend your red duration for 10 more seconds. However, not landing a Spirit Roundslash in a while will make your bar revert to zero. White will last about 6 minutes at the max, 4 for yellow, and only 2 for red, so make it count. One more thing to note about the Spirit Combo is that after each Spirit Slash (except for the three-hit one), you can press X to do an extra, normal attack. It's not only important because it adds more damage, but it also adds to your Spirit Gauge so that it stays higher, and therefore you don't have to start the combo with a near-full bar just to do a Spirit Combo. Like, you'll start the combo, expending gauge for the first attack, and gain some back with the followup, then expend more gauge for the second attack, gain a little bit back with the followup, and then use up more for the third and fourth spirit attacks.

New to MH4U that wasn't in 3U, you can combo fade slashes into a Spirit Combo. By pressing R after a fade slash, you'll do a cool spinning approach slash that also uses your Spirit Gauge. It makes you move forward, which is important for further aiding with positioning. Also, if you were to continue with pressing R, you'd go right into Spirit 3, just to provide an idea of where it inserts you in the combo. However, after your approach slash, you can also do an uppercut by pressing X before going into Spirit 3. It's different than your regular X->A->A uppercut since you can't flow into that combo from this one, but you can do a fade slash after this different uppercut to give you a second infinite combo of fade slash -> approach slash -> uppercut 2. It's a cool thing to know, since this approach slash can give you a shortcut to your Spirit Roundslash and help you land it faster, if you have less time to hit the monster.

Another important feature to note is that you can jab attack after a roll by pressing X.  Inserting these whenever possible is essential to maximizing your damage output, just like with the jab attacks during your combos.

Additionally, while with all weapons you can attack while jumping off of a ledge, it's a cool thing to note that you can attack with your Spirit Gauge off of a ledge as well, highly accelerating your Spirit Combo if you want to go through with it. It replaces your Spirit 3 attack (only does two slashes in midair though), so the very next attack is your Roundslash.