No difference. Pumping nitrogen into street tires is one of the biggest scam from tire shops.

It depends on the tire durability, your driving style, type of car, and average mileage travelled. With combination of these factors, it can last from just a year to even 5-6 years. So cannot really say change every 3 or 4 years is a must for all. You just need to observe the condition. For example, some tires tread can finish faster before rubber becoming hardened/cracking, some have treads that can last very long but rubber can start to crack/hardened. Your driving style and average distances are also a major factor in this.

For example for my driving style (faster and more aggressive than average), mileage travelled (+/- 80-100km/day), my type of cars and tires (high performance tires) usually my tires last just around 3 years or between 40-50k km.

PS definitely.

340, only a bit higher than PS4 at 320.

Yes it's fine, even as old as 2019 are still fine. They actually provide the best value for money because they're still perfectly fine as new but much cheaper clearance price just due to older manufacturing date. These kind of older tires by 1-2 years are actually my favourite when looking for tires.