Are you using both the FIIO E3 and the P4 Warbler? Or just the cable from the FIIO E3?
"Line in" and "Line out" are not terms used to describe cables, they are used to describe inputs and outputs. Based on your equipment, it sounds like you just need a mini-stereo cable that is male on both ends.
Does the cable actually matter? This is one of the most hotly debated topics on the forums. My opinion is: they can subtly change the sound. But better is totally subjective. Look up cable ABX tests on google. What you will find is that when cheap Radioshack cables are compared to cables costing thousands of dollars, 50% of audiophiles will prefer the cheap cables, and 50% will prefer the expensive cables. When asked to blindly ID which is which, almost all who participate will fail to do so at a level beyond that of mere chance.
If you play a trick on the audiophiles by telling them the $5 dollar cable is the $1000 one, they will tell you the $5 dollar cable sounds miles better. Then they'll get mad when they tell them they've been tricked.
A LOT of how a cable sounds is based on how it looks. Cables which are well constructed and look fancy will make your brain think there is added detail, better soundstage, tighter bass, etc... But try doing a totally blind test with a friend assisting, and you will 99% likely find that those changes disappear without the visual component. The power of visual placebo is a powerful thing. I've fallen trap to this many, many times!
In a nutshell: cables can slightly change sound, but which is "better" is totally subjective. Sometimes people say silver cables sound "brighter" than copper cables, but there is no hard evidence for this. No cable sounds absolutely better than another cable.
A good rule of thumb is: spend most of your $$$ on the headphones or speakers. Spend the remaining on source and amp. The cable should occupy the smallest portion of your budget.
Edited by dizzyorange - 9/20/13 at 3:00pm