Raising Rent Contrary to a Promise

When we came to sign a rental contract, the landlords (a mother and her son, who sat with us) promised us (my wife and me) that although the contract was only for one year, they would not raise the rent as long as we wanted to continue living there — just as they hadn’t raised the rent for the previous tenants throughout their entire rental period (only raising it afterwards between one tenant and the next).After four years, the landlord is now asking to raise the rent. It should be noted that the previous tenants, to whom they did not raise the rent, lived in the apartment for six years and left of their own accord when they bought a house.Our question is: does the landlord have the right to raise the rent despite the promise, or is this at least a case of mechusarei emunah? It seems obvious that the promise could not stand without limitation, but the question is whether a period of four years would be considered included in the promise (since the previous tenants lived there longer without a rent increase).Thank you in advance!

Shalom uVracha,

They are permitted to raise the rent according to Halacha.

Because even if this might be considered mechusar emunah, it is regarded as trei tarei — a case of fluctuation in the market price — in which one may be lenient, especially since rental prices have risen significantly.