boost clock they are listing means one single core can boost that high. so that speed they list as boost is onyl for a single core at one time.
overclocking opens the door to all core boosts. many can get to 5 ghz all core overclock with a HEAVY amount of cooling. like 360mm watercooling or better!! but it's possible to get an all core overclock that high if you are lucky. it is not guaranteed as every chip performs differently. but you will pay a heavy price for what real world won't be as big of a performance increase for what it will cost you to make it happen.
on top of the $400 cpu, you'll need a high end "z" model motherboard than can deliver the 250w+ it will take to do this. then you'll need enough cooling to keep that massive wattage in check.
as stated above by Math Geek, it's really difficult NOT to choose AMD at this time. They are a good performer overall and keep up quite closely with intel for a much lower price, allowing you to keep some extra money in your wallet or put that toward another upgrade in your system.
You're likely looking at a gaming rig? a Ryzen 5 3600 will suffice along side with a good Graphics Card. There are a variety of motherboards to choose from, which makes it a lot more flexible in the budget point of view, as well as getting features you want. If you roll with an AMD, you could look at the B550 boards, with newer form of hardware, which ... is not something to drool over, but definitely has a few advantage.
That being said, if you want to wait a couple of months, both AMD (for CPU) and Nvidia (for Graphics card) are releasing a new line of hardware in about 2 months time. Graphics card is a worth while wait, but the CPU is a little bit of a personal choice, it would seem it's more of a refresh rather than a BIG upgrade from the 3rd gen.
To be honest, I was just curious about the differences xd https://ca.pcpartpicker.com/list/pJGPwh here's the build I'm planning on making, I was told that the 10600k would outperform the 3700x in gaming (Plus its slightly cheaper) so I might go with that
spend all that extra cash on it if you wish, but it's not worth it at all for such minimal gains. you'll be paying over $200 more for a few fps. that could go toward more storage or a step up in gpu or just stay in your pocket.
take a look at tom's cpu comarison. you'll see what i'm talking about.
for years amd was the one to avoid but right now things flipped. until intel gets the 10 nm or 7 nm working and mainstream, they are just not worth it. they have optimized the heck out of 14nm for sure and get a lot out of it, but at 250+w vs 65w for the 3700x, it is just not worth the extra cost with the high end cooling and mobo needed to handle all that power.