These are about as hard as you get working in your head and jottings on paper may be needed to do them. Look back at earlier year groups to see the different strategies that can be used. The strategies are the same but the numbers are bigger.
Most importantly, children need to understand that 986 + 4 289 is the same as 4 289 + 986. This is the commutative law, but children do not need to know the word. In other words the sum can be done in any order.