First, round 6821 which is 6800 because the number next to the hundreds place (2) is less than five so you add zeros, 6800. Now, round 4963 which is 5000 because the number next to 9 is 6, which is greater than 5 so the nine turns to ten, it adds it to the thousands place, 5000. Lastly, add 6800 and 5000 equals 11800.
I explained so much and I hope you understood.