The bitcoin blockchain is 700 GB right now so you will need at least 1 tb which would last you a couple years maybe.  2 tb would be good for much longer.  You can order a samsung evo 2tb internal hard drive and its fairly simple to replace the 256 GB with it.  Or you could use the external 4 tb you already have.

how big of an ssd should you get?

I see one online for around 90 bucks but has a 256GB SSD

theonceler

Dell optiplex 9020 goes for around $100 bucks and downloads and syncs the blockchain in about half a day.  I used to try using rasp pi 4’s but almost always had bad experience.  I would highly recommend spending a bit more money and get a small form factor computer like the 9020.

bitcoin

MyNode model 1 with raspberry pi4 time to sync

Dell optiplex 9020 goes for around $100 bucks and downloads and syncs the blockchain in about half a day.  I used to try using rasp pi 4’s but almost always had bad experience.  I would highly recommend spending a bit more money and get a small form factor computer like the 9020.

Here’s another post agreeing with me.
https://stacker.news/items/186832