What happens when you need to tear-down a hotel but no houses are in the bank in Monopoly?
The rules state:


  When the Bank has no houses to sell,
  players wishing to build must wait for some player to return or sell
  his/her houses to the Bank before building.


and


  Houses and hotels may be sold back to the Bank at any time for
  one-half the price paid for them.


and


  All hotels on one color-group may be sold at once, or they may be
  sold one house at a time (one hotel equals five houses), evenly, in
  reverse of the manner in which they were erected.


I would say that if it is NOT possible to break up a hotel into four houses, you must sell all hotels at once, at half-face value for the entire purchase of each hotel and four houses, thus satisfying the condition "all hotels on one color-group may be sold at once".