I'd just use the usual method taught in schools, 'borrowing', but it's a little difficult to illustrate on here.

Instead you could use 'equal addition' as follows.

4010-1234

adding 1 to both numbers

=4011-1240

adding 10 to both numbers

=4021-1300

adding 200 to both numbers

=4221-2000

=2221

It's rather less efficient though.