Depends on what equipment you have available, I would suggest ELISA if you have a spectrophotometer. I have worked with a number of kits from the company Peprotech and have never had any problems, though I have never used their murine IL-4 kit. I have used their standard and mini kits for both serum (human, mouse, and rat) and cell culture supernatant and lysates. In my experience I have found their antibodies to be among the most consistent.