I believe that the RS4 racer tank does not have a built in fuel filter, but it does fit in any present nitro vehicle- exception being the savage- built by HPI.
There is some difference between external fuel filters. Some use a fine mesh to filter, while others use what I believe is called a 'sintered' fuel filter. It is basically a pourous rock material, and I feel that it filters slightly better, and it is the type of filter in my gas tank (MTX2).
If you want a cheap fuel filter, look at the Dubro lineup. They have a very uncomlicated design that is easy to clean and comes in silver, red, blue and purple. It is only like $2, and very small and compact.
I have seen some of the long and skinny mesh filters create fuel flow problems though. If you tighten them too much, the barrel-like filter can get smashed and slow fuel flow. The big fat ones like the Kyosh o filters don't have this .problem
OFNA has the only 'sintered' fuel filter that I know of. You would kind of have to go searching for it, because they make other models of fuel filters, and I don't know which is the right one.
If I said anything wrong, please tell me.