As a fellow decathlete, a couple of thoughts:
1) Different training groups have success in the dec with wildly different training methods, so it's hard to really give you good advice with no idea what the rest of your training looks like.
2) Your 100m is much faster than your 400m time. Your 400m time is much faster than your 1500m time. That should point you in the direction of what you're missing, if you hope to improve your 1500m.
3) Your easiest points increase will come from the 1500, I would think. Improving your 1500 should also improve your 400, although you aren't leaving as many points on the table in the 400m as you are in the 1500.
What are your other marks? Will you be doing your summer training at a college, or in your home town, or are you not in college? If you're in high school, the advice might differ slightly.
That being said, most training camps for the multis do at least some base stuff in the summer. Repeat 400s, 600s, 800s, etc. 2x10 minute runs, etc.
My prs in the events you listed:
7.48, 11.5, 23.3, 50.04, 4:26 (in a decathlon, I ran 4:10 in college in an open 1500 though)