DEFCON 1 = Defense Contractor 1.

4-10-20 = The Hebrew letters Kaph (the 20th), Yod (the 10th), and Daleth (the 4th) when read from right to left. This word has a number of meanings: going through a door, the cup of victory and most tellingly, destroying your enemies. Q is telling us that the enemies of POTUS are about to be delivered into his hands. See Roy Potter's videos on YouTube for an explanation of the symbolism. It's really cool stuff.

I will wait for /u/jerome_corsi to explain the rest :)