First, we find their current point in time be realizing that 4 years is 4 sets of 12 months, meaning at that anniversary they will have been together 4*12=<<4*12=48>>48 months
Then, we subtract their 6 month difference, finding they are at 48-6=<<48-6=42>>42 months into their relationship.
Since 2 years would be 2*12=<<2*12=24>>24 months into the relationship.
We take the larger number, 42, and subtract the smaller number, 24 to find the difference in months, meaning it is 42-24= <<42-24=18>>18 months
#### 18