Depending on what you are looking for, some exchanges have more options than others. Regarding MT4, some exchanges allow third-party platforms and trading bots to be used via an API, but access via MT4 is not very common (that I have seen, anyway). You can trade with leverage on exchanges that offer that. For those that do, you can usually choose the position size and the leverage amount, in lieu of being locked into a specific leverage for the entire account.
Some exchanges have fiat, but they may not be the best choice since they typically do not offer high leverage and will require KYC that usually prohibits US traders. The better exchanges also do not usually allow US traders, but they do not require KYC, and can be easily accessed by way of VPN.
The exchange does not have to offer fiat in order to cash out. You can deposit and withdraw cryptocurrency and cash out in other ways, or use crypto pay cards or some other form of purchase method to spend your crypto.
Depositing/withdrawing using crypto carries many benefits. To name a few…anonymity, fast and super cheap transfer as well as circumvention from heavy-handed regulatory efforts that may prohibit you from receiving your money. Ultimately, it affords you more control over your own money. The flip-side to that, is that you are also more responsible for securing and managing your own funds. As such, if you send your money to the wrong address or are careless with your passwords etc., then you may be at risk of losing your money, with no one to blame but yourself and no insurance in place to recover your funds.
Bitmex is probably the biggest exchange, currently, but there are others that are showing dominance. I will not name them, since you can use google to find them and learn about them. I, and many others, have stopped using Bitmex because there are better options now.