Are all tastes just a combination of sour, sweet, bitter, etc.?
Interesting question.


  The brain perceives taste or flavor as a combination of input from taste buds, the olfactory system, and even pain receptors (spicy foods). Other food contents, such as metal, spice, fat, pH, and even dissolved gas content seem to contribute. 


While there are 5 types of taste receptor, there are some 2000 olfactory bulbs encoded by 1000s of gene.  As such, it is not possible to predict how something will taste in a simplistic 5-taste combo.  However, this is an area of active research, such that structural models are used to develop taste enhancers, artificial sweeteners etc. 

Sources: 


http://www.sciencedirect.com/science/article/pii/S0976120913000041
http://health.howstuffworks.com/mental-health/human-nature/perception/taste.htm
http://www.livescience.com/17684-sixth-basic-taste.html