Things to note:
A 2X4 is actually 1.5" X 3.5"; a 2X6 is actually 1.5" X 5.5"; and a 2X8 is actually 1.5" X 7.25".
Making a beam made up of two boards side by side is twice as strong as each of the individual boards. For a 48" span, a 2X4 supporting 1000#s deflects 0.269" while a 2X6 only deflects 0.069" and a 2X8 deflects 0.030". That said, it is much better to go up in size then it is to double up.
When assembling the stand, be sure all of the pieces have square cut ends and that they are as straight as possible. If one of the long boards for the top frame (red above) has a crown (bows up when stood on edge), be sure to place the crown up. Glue will help bond the pieces together and coated decking screws will resist rusting. Clamping the pieces together and pre-drilling the holes will help make assembly easier.
Pocket drilling the legs (purple) can eliminate the need for the screw strip (green) but I would make sure to skin the stand on three sides to keep it from racking/twisting. Also, be sure to screw both leg pieces together on the corner for added strength.