Since it seems like a lot of folks are coming in to this thread with very little knowledge of PF2, here’s the rundown: at the core of PF2 are two major innovations: the three-action economy and degrees of success.
Degrees of success are fairly simple. As with most d20 based games, you have your target number, your d20 roll, and your bonus. If your total meets or exceeds the target number you succeed. But in PF2, if you exceed the target number by 10 or more, you critically succeed. If you undershoot the target number by 10 or more, you critically fail. A natural 20 upgrades your degree of success by 1, and a natural 1 downgrades it. The system plays with this in interesting ways, for example, most spells that deal half damage on a successful saving throw now deal no damage on a critical success and double damage on a critical fail.
The three action economy though is the real masterstroke of design. Instead of categorizing different actions as “standard”, “minor/bonus/swift/whatever”, or “move,” everything is just an action, and you get three of them. You can draw your weapon with one action, move your speed with another, and attack with your third. You can use two actions to move your speed and one to attack. If you really need to cover a lot of ground you can use all three actions to move your speed. If you’re in the thick of it and just want to wail on your target you can use all three actions to attack. That said, it’s Pathfinder, so your second attack in a turn is made at -5 to hit and your third attack in a turn is made at -10 to hit. You also get one reaction per round, exactly like 5e.
The three-action economy is the backbone of the spellcasting and maneuver systems. Power Attack, for example, is a Feat that lets you use two actions to make a single attack, which does 2[W] damage if it hits. There’s a charge feat that lets you use two actions to move double your speed and attack. Casting a spell requires one action for each component (action for verbal components, action for somatic components, action for material components), which is used to help balance more powerful spells by making them cost more actions. Metamagic allows you to add effects to your spells by adding components (also a balancing factor because you can’t add, for example, material components to a spell that already requires them.) Some spells have Metamagic-like effects built in. For example, Heal (PF2’s equivalent of Cure Wounds) lets you restore hit points to a creature you touch (or damage an undead you touch) with one action, lets you affect the target from up to 30 feet away with two actions, and affects each target within 30 feet with 3 actions. Magic missile shoots one missile for each action you spend casting it (and one for each spell slot level above first you used on it, 5e style.)
Where the 4e comparison starts coming in is with character advancement. Every level, you get a Feat. Some levels it’s a Class Feat, some levels it’s a Skil Feat, some levels it’s an Ancestry Feat (they call races ancestries), some levels it’s a General Feat. Some levels you get Feats from multiple categories - especially Rogues who get twice as many Skill Feats as everyone else. But Class Feats especially look pretty much like Powers from 4e. Your Class is basically a big list of
PowersFeats you choose from at certain levels, most of which give you a new special attack you can make by spending a certain number of actions on you turn, like Power Attack mentioned above. They are also laid out almost exactly like Power cards in 4e. Here’s what a page from the Fighter class looks like:
Unlike 4e, there’s not much in the way of Encounter or Daily Powers. Most martial Feats are at-will and balanced with the action economy. Spells are daily and Vancian. There are, however, encounter powers of a sort in the form of Focus Abilities. Everyone has a pool of Focus Points (I think they’re based on your Charisma?) which are an encounter-based resource you can spend on certain abilities, most of which are preternatural in some way. A monk’s ki abilities, for example, are Focus based, as are an Alchemist’s infusions. Many casting classes have a few Focus spells in addition to their normal spells.
