Jaysan,
4 cukes in an 18 gallon container is no issue. I always plant 12 cukes (or more) in my 31 gallon EarthTainers - and they thrive. Yes, there is a bit of "competition" for sunlight as they get up to about 3 feet tall - but then I thin out the weaker vines.
Density is not your issue at the moment, instead it is likley soil or nutrient related as others have suggested.
I believe it is normal for about 25% of the flowers to then not fully develop into cukes - just the plant's way of self-regulating propagation instincts.
Raybo