Addition of two 4-digit numbers is usually done on paper, using the standard method described on the worksheet. Once addition of HTU has been mastered it is a fairly straightforward step to adding thousands, eg
5 6 8 7 + 2 5 4 6
The key is to keep the numbers in line (tens under tens etc) and to always start with the units. Most children will not have a problem with this and many really enjoy completing sets of sums. It is a good idea to encourage checking by doing the inverse ie subtracting the second number from the answer to give the first number. This checking could be done on a calculator.