Hi,
Yes that scaling would work, the 4W rating would have been calculated under laboratory conditions, so your scaling should be pretty accurate.
The resistors will get hot with 2W dissipated in then, the wattage rating is for how much power the resistor can tolerate before malfunction.
You will notice they haven't burnt out like the smaller 1/4W resistor did.
If you are going to make a board or something to mount your components on, it would be better to mount the load resistors up of the board or base by 15mm or so to allow air to flow around.
If you can get hold of a 10W 10 Ohm resistor that would be ideal, but you would still need to mount it so air could flow around it.
Looking at your results, 10 Ohm or even 8.2 Ohm might be better.
Your project looks good as a prototype. Well done.
Tom.. 
