8k prestiges as Viking is too easy, 8k prestiges as Catholic is way too shabby...
But well that requirement is there to avoid Europe mainly for getting too many Charlemagne wannabe left, right and center...
So from this little information let's summarize, you've got 4 kingdoms title which means 4 prestige/month obviously...
14 x 12 = 168 prestige/year, so assuming you go from say 2.000 prestige as starter, means 6.000/168 = roughly 36 years to do up till 8k prestige

Without an ability to raid, it's almost impossible to get 8k prestige as Catholic except you joined Crusade and won then BAM! 5k prestige.
What you can do though to make this dream of yours slightly come true is :
1. Creating ALL ducal title available to you, 1 title net you 200 means 5 titles can net you 1k, good isn't it?
2. Befriend Pope to excomunicate and then declare war to targeted excomunicated char, or better yet, create an Anti-Pope and make him do your bidding to milk prestige.
3. As opposed to point number 2, helping the Pope dethrone an Anti-Pope (if there is any), will net you many prestige.
4. Take a War focus and duel every single one of your rival/excomunicated char, each 1 duel won will help you net 200 prestige, well this one is little bit risky though...
5. As Catholic, do a Feast or Summer Festival every year, as a King of 4 Kingdoms, you must have ton of money ain't that right?
6. Fight a tributary war against your neighbor to net prestige and forced allies advantage as well.
7. Having someone from your line of Dynasty as Head of Holy Order or better yet the Pope will net you extra +2 prestige, From "Dynasty Control Holy Order/Catholic religion" as well as 1 piety, similar to Caliph and all his kinsmen. Sadly this need a tremendous luck and unlocked Crusade...
8. Helping your co-religionist fight defensive war against heathen will help gainign prestige, from your description and Kingdom titles, I smelled Ummayad nearby...

9 Taking Carousing focus and have party every 1,5 years also help, even though only a little, but this is much safer than War focus, except you're accidentaly maimed when fighting a brawl
And so many other ways that maybe other forumites can also help, good luck.