Becoming curious about your question, I looked up your bike on Bicycle Blue Book. The specs listed here show the '14 Trail 4 to have a 1.5" straight headtube (see section on specs titled, "Frame"). If this is so, then you can use forks with various combinations of steerer tube sizes like 1⅛" straight, 1.5" straight, or 1.5 to 1⅛ tapered.
This section from the Cannondale Answers website has the information and links that will clear a lot of things up regarding your options.
You need to measure the internal diameter of your headtube by removing the fork and, using calipers preferably, determine what size exactly is your bikes headtube. If the internal diameter is 49.6 mm, you have a 1.5" headtube. You may realize that 1.5" is 38.1 mm. The additional 10.5 mm of diameter is utilized by bearing width. It seems that you can more easily determine if your headtube is 1.5" by leaving everything together and measuring the height of the raw headtube. If this measures 134.6 mm, it's likely you have a semi-integrated frame (the cups for the bearings are machined into the frame, i.e. there are no external cups. These headtubes are 1.5" and are open to the variety of steerer tube diameters mentioned above when the appropriate headset is used. This information is outlined in the Cannondale link above.
Finally, if you find that your headtube is, in fact, 1 1/8" and your choice of steerer tube becomes limited, there is still a wide selection of suspension forks with threadless 1 1/8" straight steerer tubes. While the tapered and oversized (1.5") steerers are taking over the market, my feeling is that 1 1/8" straight steerers remain the most commonly available option. Utilize filters on your favorite bike parts websites to limit the choices of suspension forks to those available with 1 1/8" straight steerers.