Pronunciation differences between pinyin 'zh' and 'ch', and between 'qi' and 'xi'
The difference between zh and ch has nothing to do with the position of the tongue: these two are pronounced with the tongue in the same position; the difference is aspiration -- ch is aspirated while zh is not. Both are retroflex affricates.

x and q have less in common -- actually q and ch are much closer to an untrained ear, as they're both aspirated affricates -- the only difference being the position of the tongue, ch being a retroflex, and q an alveo-palatal. x on the other hand is not aspirated, and a fricative. Actually x is close to the German Ich-laut (except ch in Ich is a palatal rather than an alveo-palatal).