I've done that myself over the last month or so. On an external drive, I installed PostgreSQL manually, and then followed the instructions, noted down pw and port number and then installed PT4. It is quite seamless, but you need to install a compatible (less than the most recent) PostGreSQL.
It works very well, although this setup isn't the ideal case of a large SSD: the external is an HDD and my internal drive on this computer is an SSD but does not have enough space.
My old DBs amount to something bigger than 12GB, so I can't even do analysis on my playing computer which is a laptop with far less space remaining than that - another small SSD.
So as it is, I will have to regularly move my hands from the laptop into the main DB, but that's OK.
I can look over a long period of time and see how I went from fish to slightly less fishy.
Now that I'm taking my poker more seriously, I like looking at reports like population tendencies, scatter graphs and so on which I never did before if you can believe it. PT4 is definitely among the best investments I ever made to learn the game.