If you've been suffering hamstring issues then you need to look at why that's the case…….think about whether you're doing weights and what kind of exercises you are doing that actually use the hamstrings. Many sprinters forget about them and try to run mainly using the quads – problem being once you get faster you need everything to be as strong. You'll generally find the back and the hamstrings are also very closely related fitness and injury wise and one could be cause the problem in the other area as well.
As for training, you look like you have the basic raw speed there for the 400 times you're after so it's time to knuckle down to some slightly longer hard runs – anything from between 200-500 and I'd suggest the majority of the being between 200-300. Without having seen you or knowing you its difficult but I'd suggest you try a few sessions along the lines of 3×300 with full recovery between each, 300-250-200-250-300 with say 8-10 minutes between each.
As for coaching – get in touch with the sprints coach at your regional high performance centre – there may well be another coach close by that you don't know about.