i have a 4x. It has been the best in its class. Now that there is a new generation of cars, it is obviously older but still competitive. For a non pro driver i don't think you can make an argument against the 4x. Buying parts or a complete car on ebay is half the price of a current car. Its fun owning the latest but unless you are racing against the top world drivers you should be fine with the 4x.
the 4x is probably more durable. the 5 has new ways to adjust the set up of the car. its a different car altogether. the 5 has also been known to have radio problems, possibly from the receiver box. the 5 also has a new clutch design.
Irwin RC 1/8th Pan Car Specialists - www.irwinrc.com
SPM Competition, WRC Racing, MIP, Novarossi