To improve your 40 you need to work on a couple things...

 Explosion. That first 10 yds is super important because it gives you momentum to propel you to the finish line. You can’t just stand up and run. You need to drive out for 5-10 yds. That’s why they call it the drive phase. Working explosive movements in the weight room will help with this. Power cleans, jump squats and plyometrics is what I reccomend Top speed. This is that 10-40 yd range. You need to work resistance running. Hill work, sand work, and band/sled work will be best. Form. Perfect arm swings and proper leg drive, foot strike and pad level will all help your time improve. Arms at a 90 degree angle, swing from your shoulder. Keep your ankle flexed and strike the ground with your toes, drive your knee up to a 90 degree angle almost.