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// The Answer 18 
// Story: Life is A Test 2: Test Harder (Quite A Bit Harder) 
// by Brony_of_Brody 
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You only need one trip there and back again.

First, label each end of the cables on one side A, B, C and D. Then, connect any two of them (let's say A and B for simplicity) and go around to the other end.

On this end, label the cable ends on this side 1, 2, 3 and 4. Test away pouring power into each until you hit a complete pair which forms a circuit. Let's say it's 2 and 4.

From there, disconnect one of them and attach it to another unused cable - let's say 2 to 3 - and then go back to the other side. Disconnect A from B and test again for a different complete circuit. One of them will be A or B (the other end of 2) and C or D, the other end of 3. Let's say it's A and C. 

Now, you have all the information you need to match up the rest. A is 2, so B is 4, C is 3 and D must be 1. This works regardless of combinations that form circuits.