What year was my Winchester was made serial number 4099304

Mark, I'm not sure if you ever got your answer in all of that above but just in case you didn't....yours is in the war years 1944, or 45. Winchester kept no records of public serials those years because pretty much all of their manufacturing went to the war effort. Simply no record keeping. In 1942, #1221289, was sold. 43, 44, 45, 46, and 47 there were no records. In 1948 they began again at the number 1500000. You can't get any accurate to the day date on yours but if you divide those numbers by the five years you get approx 55,700 a year. So 1221289 + 55700 is approx 1277000. Add another 55700 and you get approximately late 1944 to early 45 for your rifle. I hope that helps.