You desktop is probably consuming around 60-65 watts idle (depending on the efficency of your PSU). I have a computer that I build a few months ago that does the exact same purpose (downloading, file server and some backup duties) X2 245, with intergrated 4200 graphics, 2x 2.5" hard drives and 1 SSD and that pulls about about 40-44 watts (I undervolted the processor).
Yours will idle higher than mine because of the quad core processor, and the 3.5" hard drive.
So at 60 watts for 4 hours 2AM-6AM) it is consuming a total of 240 watts.
A netbook will probably run about 10-20 watts but it varies on the netbook (more specifically the specs), my full laptop pulls about 11-12 watts idle and 25 load
2. Is your goal to save money or save the planet?
If your desktop uses 75watts an hour @ 15¢ per kWh a 4 hr download session costs about 4.5 cents.
A netbook @ 15 watts @ 15¢ per kWh that 4 hr download session costs about 1 cent.
Saving 3.5 cents a night running 365 nights is going to save you ~$12.77 a year. Divide the cost of your netbook by $12.77