The twin dipole arrangement gives greater signal strength, and allows you to 'steer' the antenna.
The dipoles on my setup are 6M apart, and the masts are 3M high, but 4 should be ok.
There is a lot of useful info here:
http://radiojove.gsfc.nasa.gov/help/help.htm
and the antenna construction manual is here in pdf format:
http://radiojove.gsfc.nasa.gov/telescope/ant_manual.pdf
Heres a pic of my setup:
www.aussiesky.net/misc/Jove_antenna1.jpg
Greg