Can my headtube take a tapered fork? I'm trying to replace a Suntour XCT fork on a '14 Cannondale Trail 4.  

I think it's a got a 1 1/8" straight headtube looking at the specs here: https://www.vitalmtb.com/product/guide/Bikes,3/Cannondale/Trail-29-4,13741

On Ebay all the forks seems to be tapered. I.e Rockshox Reba or Judy.

Looking on CannondaleAnswers here - https://www.cannondaleanswers.com/definitive-guide-cannondale-frame-headset-fork-compatibility-for-mountain-bikes/, it shows that a conversion is possible for some headsets with a KP205. 

The issue is I don't know what I have! the headset has 'Tange Seiki' on it. Does that mean its SI integrated? 

Anyway advice is greatly appreciated! we are both new to mountain biking so have very limited knowledge!

Thanks.
'Tange Seiki' is just the headset manufacturer.

Reading this https://www.cannondaleexperts.com/Si-Mountain-Headsets_c_1569.html it seems that Cannondale SI Integrated headsets were used on Headshock and Lefty forks, so there's no reason why your bike would have that type. 

SI bearings are pressed directly into the frame so if you have separate bearing cups they are definitely straight 1 1/8.