I tend to get more life than that out my chains, but it's not surprising to me that you need a new chain after 4000 km miles. Chain life depends on the chain, how often it was cleaned and lubricated, your riding conditions (wet vs dry, flat vs hilly) and your style of riding. A lot of people change their chain every x number of miles. Some measure the chain for wear before replacing. Regardless, it's a part that you can expect to replace pretty regularly.