Is using 4-pin connector for 8-pin ATX 12V power connector ok?

IT depends on your CPU. The additional 4 pins were added to support the higher power draw of newer (at the time) Core 2 Quads. The 4 pins will happily provide power to most lower end CPUs, but if you have a Quad I would double check the TDP and make sure you can provide enough power with the 75W that the 4-pin can provide and that you don't need the 150W that the 8-pin provides.

Either way, I would upgrade your PSU. You can get a good, brand name, high efficiency PSU with the proper connectors for well under $100 and you will be all set in the event that you upgrade down the road.

Edit: The Rana has a 95W TDP, this means that your CPU will consume 95W on max load. Your 24 pin power connector provides 144W max on the 12V rail. 75 of these Watts are used by PCIe, which leaves ~69W for the CPU (though other components will use some of this so in reality it's probably closer to 60W). This means that with the additional 75W from the 4-pin you can ~135W available to the CPU under max load. This should be plenty assuming you are doing no overclocking and your motherboard and you don't have any other major 12v draws in your system (like an additional video card).

Seriously though, you should still get a new PSU. They can be reused in newer computers if you upgrade and there's no guarantee that you aren't voiding your motherboard warranty even though it should work.