Just on my point, OP can serve a notice now to end the lease in 4 years. The tenants may believe they are on an indefinite lease and can do what they like, including being late with payments every month. If you serve them a valid termination notice it becomes clear that they are going to lose something through their behaviour, even if it's in 4 years time and that might encourage them to pay on time, if you tell them it isn't written in stone and you might reverse the notice/let them stay if they pay on time.
The other advantage of legally serving now is to avoid the case where they change legislation in the future and you lose the power to terminate the lease. I'm not a lawyer but would guess the legally served notice couldn't be invalidated retrospectively.
The issue with any termination threat is that in 4 years the lease rolls over to an indefinite one and OP loses the leverage. OP could also inform them that if they don't pay on time they will sell the place or move back in. The issue there is that they may bring in selling with tenants in situe and you lose that leverage and are stuck with them forever. This type of situation, combined with rent controls, is why a lot of people are selling up.