I'm just curious if it is possible do it. I have a spare 4gb sd card.
Because the bandwith of the SD card interface is quite low compared to even a regular HDD (around 20Mb/s compared to 70-100Mb/s) it will be much slower. Where you can gain speed is by adding as much RAM as possible, and using an SSD instead of the HDD.
The question is about using an SD card to store frequently accessed files caching these in de SD card instead of the pagefile... That's Windows stuff in fact. Windows uses a special algorythm amd compression to do that. However: Even in our "beloved" MS operating system ReadyBoost does only provide a very limited improvement on realtively up to date hardware and it is more a way to speed up older drives.
I know that your question is not about MS Windoze, but before giving an answer for Linux I had to exaplain what the hype is all about.
Linux does not work like Windows (and neither Mac OS X for that matter) and the key here is memory management. All this readyboost stuff just isn't applicable and it doesn't make much sense. In Linux the closest thing would be using the SD card as extra swap... you can find it here in Lifechaker
BUT before doing that I would encourage you to read the comments.
So, maybe your 4GB SD card may be better employed for other stuff such as putting a little virtual machine into it or holding a small read-only rescue system.
BTW: In Raspberry Pies this stuff seems to make actual sense ;)