Am I right in understanding that the core will effectively be 1/2" in the middle tapering to 1/4" at the edges? If you want to save some weight and cost skip the layer of glass between the two layers of foam. That layer is at the lower stress "neutral axis" area, and will do very little for you.
A good rule of thumb for basic E glass is 3 layers of 6 oz each side no a half inch core. If the board is wide and you're not huge you could get away with a bit less glass. Now, if you go S-glass sounds like your layup is about right using the 4's. If this is your first board, I'd suggest 3 layers of 6oz E, and drop more cash as you learn a bit more.
Your original plan had 32 oz total of S-glass, includuing the middle layer. You'll be at 36 oz with 3 layers of 6 oz E each side, so not a huge weight difference. Unless you plan to really stress it, go with 2 6's and a 4 each side of E. Or, if you can find it (like if you're in Canada getting stuff from Composites Canada) get the 7oz style 7715 E glass, which has 90% of is strength in the long axis and replaces 2 layers of 6 oz for a better price than S-glass.http://www.compositescanada.com/product
... roduct=613 (note that the description on that page seems to be wrong, the same as anothe type of cloth, but the pdf link on that page has the right info.)