These vaccines are developed to be used in a field/barn situation and the manufacturer has to state a time frame so that they cannot be sued....
If you use a single sterile needle, withdraw into the syringe and then keeep the initial needle in the bottle for next dose etc etc, do all your four doses into different syringes in your house before being in the field, them immediatey place tape over bottle and put back into fridge... better still if you plan longterm and get an injection gun, store that with bottle connected in the fridge... obviously no guarantees that is doesn't fail, but to be honest you never have that...
If you calculate the risk for 4 sheep over a period of a coupe of years... that's quite a few bottles at 20odd quid a go.