It's hard to evaluate your exposure without knowing the lighting conditions, but f/4 at those speeds in almost any kind of daylight (excepting very early or late and low sun), per film, should be enough, if not too much (as your Delta 100 "a bit dense" indicates). Your HP5 would be flat if under exposed far enough, since even the highlights would be weak, thus the whole thing flat. Development usually controls contrast, but you must be reasonably close on exposure to evaluate development.
So - Ian could be right about your shutter. A standard check would be to shoot a set of negs, all same subject and lighting, on tripod, consistent lighting. Start with a good exposure on one end of the shutter speed range, then go through the speeds, changing the f/stop each time to compensate for the speed change. Develop all the films together. If there are wild swings in neg density, you have a shutter problem.
Also, you don't mention using a light meter. Without one, and without some experience, you could be off.
