Perhaps you could tell us what your computer is?

Motherboard or computer model numbers?

Lenovo thinkcentre m70e. R8AY95G serial number. Dont know motherboard sorry. CPU is dual core e5700 3ghz.

If you google the phrase below, you should see offers from people selling the ram that you need:

Lenovo thinkcentre m70e ram upgrade

So it looks like it uses a earlyish DDR3 chipset and DDR3 at a less common clock rate because of that.

You may find you can just use standard DDR3 and it will down clock (indeed Crucial's Memory Finder indicates just that, as it says DDR3L-1600 is compatible)

That aside, given 4GB is the maximum supported and it's a VERY old PC, you might want to think about replacing it? 

PBTech For instance will be selling entire similar age systems, with 4GB of RAM, for $49 on Cyber Monday, or i3 4gb systems that are several generations newer for $149.

DDR3 has gone up in price, 2nd hand and new, as others have mentioned, maybe its time to look at a newer 2nd hand/recond  box with newer tech for probably same price as you'll pay for DDR RAM. Im in same sort of situation, great system, 8GB but Id like to push it to 16GB, but money Id spend on the RAM would be a good start on a new system.