The amount of money they currently make in a year is 12 * 4000 = $<<12*4000=48000>>48000.
The additional amount they need to make is $60000 - $48000= $<<60000-48000=12000>>12000 per year.
So, they should make $12000 / 12 = $<<12000/12=1000>>1000 more every month.
#### 1000