This is a great help to know its really easy for the future - but i'd still stick to my opinion for now, mainly down to cost alone.
A Seagate 4TB external hard drive (sponsored by PlayStation) is £75, whereas a 4TB SSD is £500. Yes, £500. Even if you want just a small upgrade like 1TB, that still would cost £150 whereas the cheapest external 1TB you can get to work with a playstation is £38.
Until the market is saturated with many AAA PS5 games, I'm really not sure it is needed yet. We need to get to the stage where SSD is available for less than 3 figures before using methinks.