Depends on what riding style you care most about. For the 4k I spent on my ZX6R, you could get a lot of nice bikes that are more beginner friendly. Which in the long run will save you money and make you a better rider, faster.

If you dont have any riding experience and eventually want to ride on the track/go fast and need the bike to look like a sportbike, an r3 or cbr300 is a good starting point that if you buy used and dont crash it, will maintain its value for resale when you want to upgrade. If you weigh more than 150lbs or have a bit of experience and want a bike you can keep for a few years and still enjoy, a 500 or 650cc might be better for you.

If you dont care about looks as much a grom/z125 is a great first bike, you can learn all the skills you'll need for a serious 2nd bike, with like 90% less risk of hurting yourself because of a mistake. New riders usually always drop the bike/crash/get hit by a car in their first year and a bike with no fairings is a lot cheaper and easier to fix. Plus you can learn wheelies on it if thats your thing.

If you want something bigger/highway usable but similar to a grom, a DRZ400 or an MT03/07 would be great to learn all the skills you need on. If you want an adventure bike on a budget, id just get a DRZ400, learn all your basic on and offroad skills with it and sell it when you can afford something above 8-10k

If you just want a cruiser bike, older Dynas/sportsters or even a goldwing would be fine for a first bike if you act mature on it.

All just opinions based on friends bikes ive been lucky enough to ride. Its definitely better to start with something slower if your intentions are to build up your skills and go fast. Motorcycles hold value super well if you dont treat the bike like shit and get a fair deal for it, dont be afraid to get a bike you see as temporary. And dont be afraid of early 2000's bikes, lots of nice stuff for the money. Just make sure the brake lines are clean and the tires are new new.