You want a 1.5 pound plane to carry an additional 2.5 pounds of payload? You will be asking a lot of this airframe. Your wing does seem to have a very short chord and span to carry 4 pounds. Your wing area adds up to 231 sq inches or 1.6 sqft. This would give you a wing loading of 15 oz/sqft empty (very light and fliable) and 40 oz/sqft with your 2.5 lb payload (a brick with wings). Sure you can get it to fly with sufficient power and maintaining a pretty high airspeed. Then you may run into structural problems with a model this small. Try to get your wing loading not to exceed about 25-30 oz/sqft. You can increase the wing size or reduce the payload to accomplish this. To fly 4 pounds with a 30 oz/sqft wing loading you would need a wing with about 307 sq inches of surface area or 2.13 sqft. Increace the wing size to 6.5 inch chord and 48 inches and you'll be in there. I am also assuming your fusalage length is just the box up front and not the boom (This overall length should be included). For reasonable stability you will want at least a 36 inch fusalage (including the boom) but a longer tail moment will be more stable at about 40 inches. This is just a quick assesment, I hope it helps.

EDIT: I thought I should include the formula for calculating wing loading.

Wing loading = Flying weight ounces / (Chord inches x Span inches /144)