What external drives work best with Mac mini M4 Pro?

You should be able to continue to use your existing drives without any ill effects.

While you can certainly get faster drives and even faster enclosures, if they work on your current Mac they should have no issues on a new Mac mini.

Beyond that, what is your question? Are you wanting to know if you need to buy drives?

You don't really.

But you can buy pretty much any hard drive out there and it should work.

If you want some high quality and fast drives, look at OWC.

For example:

click here ➜ OWC 4TB Express 1M2 Portable NVMe Thunderbolt (USB-C) SSD USB4 Ultra Fast External SSD Drive with Aluminum Heat Sink Enclosure - Amazon.com

For something less expensive:

click here Crucial 4TB 2100MB/s Thunderbolt 4 / USB-C SSD - Amazon.com

As @Phil0124 had stated, the old drive should work just fine. As far as speed, if there weren't any issues with speed on your old MacMini in terms of slowing down your workflow, there won't be any impact on the new Mini.