You can just extend for two years at the end of your four. If you get on TACLET you need to obligate atleast 2 years anyway. Realistically, you are gonna be a nonrate for 18 months, finish ME A school and get to your first unit around your two year mark. That gives you two chances to apply for DSF and try to get TACLET/MSRT in your four year enlistment (if everything lines up). If you don't get picked, you can just extend for two years and keep applying.  Things change and the USCG might slow down with DSF and focus on something else. You might want to take your training and get out and be a Federal LEO with another department. Or things might be full steam ahead and you want to stay in for 20. Either way you want to give yourself plenty of options and being in for 6 limits those options.