Four seasons hasn't borrowed the money, so you can't demand it...

Four seasons hasn't borrowed the money, so you can't demand it back from them. They are the lessee. REU has lent money to the landlord, not the tenant.

REU's loans are not the only loans against the property, and their's is the last tranche, ie if the loans get called all of the other mortgages will be paid out first.

(As the most subordinated tranche, REU got higher interest payments for the higher risk. In fact, that was how they intended to make money. Borrowing cheap and then lending at higher rates.)

They can't just ask for their money back from the borrower, because the assets aren't worth enough to pay back their loans and there is no other security, such as guarantees. Selling the properties will pay back the more senior lenders, but not REU.