Just because the frequency is twice as high doesn't make it twice the machine. The Tracker IV is a beep and dig detector that's relatively inexpensive due to it's simplicity.
The more control options and better processing abilities are what increases the price (along with special considerations, like multi frequency or gold frequency).
All detectors have their strengths and all have their weaknesses. The best thing is to study both and find what makes you feel is worth the investment. When you limit the price to $500, you take out many detectors yet many good units are still in the running.
Right now, one of the best deals for a sub $500 price is the Teknetics T2 Classic. Coming from a Tracker, it may even be a bit too much.
A used AT Pro is a good detector, especially if you want to relic hunt. The Iron Audio, expanded iron range, etc. helps in target ID. If you just want coins or jewelry, my LRP can keep up with my friends AT Pro for less cost.
When you say you want to "move up", that doesn't say a lot. You could get a Tesoro Vaquero and be moving up. Almost literally twice the depth of the 6" Tracker and ground balance.
People can throw names out at you, and a Whites fan is going to say maybe CoinGT or TreasureMaster Pro because it's a Whites. You may even get a Go Find 60 thrown in from Minelab fans. Or even asked to wait for a Ace 400 by a Garrett fan or a used AT Pro. Personally, I would by new for the warranty and resale ability by trying before you fill out the warranty card.
It's hard to A=B detectors side by side (unless you live in Orlando). Most of us watch video's, talk to others with experiences and work with good dealers (which TNet has a lot). Ask about specifics, and not just a land detector. Do you hunt at night? In the rain to dig easier? Do you want to hunt relics? Coins? Jewelry? "All types of treasure hunting" could mean Meteorites, rolled coins, flea markets, etc.
Good luck, and I hope you get the right detector the first time. It took me many tries with almost a dozen over the years before I settled. Which is why I am writing this.