I love massaman, green curry, red curry, yellow curry, etc.   Can any of these be made quickly at home with only 4-5 ingredients? 
I think @roux is generally right, curries are like mexican moles, they have lots of spices and are fairly complex. But I do think there are some short-cuts.    The most important components in a curry are sweetness, creaminess, heat, citrus, salt, and depth.  I don't know if by simple you also mean you want to use common ingredients, or just 'few' ingredients.  This is what I would use if I only had common ingredients:  Coconut milk, cayenne pepper, onion/garlic/ginger, lemon/lime juice, salt/pepper, sugar/honey.  I'd also add basil/cilantro/jalapeno for green, tumeric/cumin for yellow, ketchup/tomato paste/chili powder for red.    If you have them, the traditional ingredients you're trying to replicate are lemongrass, lime-leaf, red or green chiles, and fish sauce (or soy sauce). 