I think the answer to this is really simple but right now I cant see the forest for all the trees

consider the update statement

UPDATE [MyTable]
SET [Field1] = @Field1,
        [Field2] = @Field2,
        [Field3] = if Field4 is true then [Field3] else @Field3

In other words if Field4 is true (it is a bit field) then I do not want to update Field3, but if Field4 is false then I want to update Field3 with the passed parameter for Field3.

Any help would be awesome