Mmm.. Just get another DDR2 667MHz laptop RAM module. Should be easily found.

It will run fine. There will be a chance of the memory modules not being compatible with one and another though, but it rarely happens. If it does, you can always return to the shop to request for a different module

Here are the steps that you can use to troubleshoot or solve the issue you're facing:
1) Update your motherboard's bios
2) Enter your board's bios and manually set the RAM modules parameters to 1333MHz for memory frequency and 9-9-9-24 for RAM timing.
3) Google for Memtest (a RAM testing software) as well as for the tutorial on how to use it and use it to test the 4GB module you have
4) Run only one RAM in your system and on the slot that seems to be giving you the issue to see if it's working normally. If there is any issue the problem may lie with your motherboard's RAM slot.

Kingston offers lifetime warranty for their RAM modules. The definition of lifetime warranty for the RAM module is for as long as the modules are still in production.

You can send the RAM back to the shop you have bought it from. Alternatively, you can bring it to a Kingston products distributor like Amble Action, though I would advise that you drop them a message first to see if they accept returns from end-users.

The production for the RAM you have stops when the manufacturer announces so, or when the market starts focusing entirely on a new generation of RAM, e.g DDR4 chips.