Pretty much what I figured, just figured I'd give you a heads up in case you felt like having a headache :D

I took the same route with sieve of E and also tried atkins sieve, but thanks to this comment I was able to clarify a guess I had and the solution was obvious. Frustrating as hell to work on, but I can say that it's definitely a lesson learned when working with large numbers and what the JS engine can handle. Thanks for making me learn :D