Retroactive redemption of neta revay/observing orlah laws

Question

We just made aliyah and are renting an apartment from a non religious family that has lots of fruit trees. The trees are all more than 6 yrs old. I don't know and can't ascertain whether they observed any of the halachot of orlah. Since the trees are more than 4 yrs old, do I not have to be concerned or is there still something we need to do retroactively?

 Anything the trees produce this year we will be mafrish terumot and ma'aserot ourselves as they have given us permission to collect the fruits from the garden.