The category denotes the height of the river at it's centremost (or deepest) place. A category 2 is between 9.2 and 9.6 metres high , a category 3 between 9.6 and 9.9 metres and category 4 10 metres plus.
I presume a category 5 would be up to around 10.5 metres but at that point the river would overflow many of the embankments. I understand that this has happened in the past but I wasn't here to witness it. Further facts and times can be found here: http://www.severn-bore.co.uk/2018_times.html.
Cheers