I'd stick with strength. With getting half a level in natural gains in speed and agility, 4th capping it is just a waste of SPs IMO. Soft-cap strength first. Then, I'd half-consider soft-capping tackling next before going on to vision because it, like speed, agility, and strength, gets half a point in natural gains. Vision, on the other hand, only gets 1/5 of a point in natural gains.
Only centers, guards, and DTs (kickers and punters don't count. lol.) get more bang for their buck with natural gains. Sadly, most people ignore this and up wasting dozens of SPs on other skills (like vision and confidence) relatively early in their player's career. Think of it this way: once you reach the soft-cap in a DE's major skill, each level's natural gain is worth an ENTIRE SP (if you want to think of it that way)!!! At the second cap, each 0.5 natural gain is worth 1.5 SPs (again, if you want to look at it that way).
Here's how I'd spend my SPs if I were you:
1. Soft-cap Strength - Weaker DEs are going to get pancaked pretty often this season, if the preseason games are any indicator. Plus, getting this soft-capped earlier rather than later will save you TONS of SPs down the road.
2. Soft-cap Tackling - Self-explanatory, really. You'll be a great pass-rushing DE, but powerbacks are still the name of the game right now, so you'll definitely not want to get trucked when you do face the run.
3. Hard-cap Strength
4. Hard-cap Tackling
5. Soft-cap Vision
6. Hard-cap Vision
7. Soft-cap Confidence
Frankly, I would hold off on confidence, vision, and stamina until you've got strength hard-capped and tackling soft-capped at the very LEAST. This will give you plenty of time to train those 3 as well.
Training:
1. Vision/Confidence
2. Confidence/Stamina
3. Stamina/Jumping (Only train this after the other 3 skills are fairly well developed. This can help you swat balls and make diving tackles)
Hope this helps!