Hard to answer such questions exactly with so little 'parameters': Temperature, Elevation, Distance Dest-Altn, etc. etc.
20 tons of Load seems a bit optimistic to me. You would have to have a VERY light A/C and the increased MZFW-Option at the same time.
Our DOW (with Cargo Loading System, Brake Fans and a lot of other stuff) is about 44.000 kgs and MZFW 61.000 kgs which would give you a max load of about 17 tons.
4 Hours is about 10 tons of Tripfuel plus 2,5 tons Hold and Altn Fuel (optimistic) would be a T/O-Fuel of about 12,5 tons. 61+12,5 = 73,5 which is our MTOW, however optional MTOWs are available up to 77 tons for the A320.
About the TOD let me get back when I find the power cord for my laptop.
Regards MAX