If the document creating the trust specifies some rule or process for decision making, the terms of the trust instrument must be followed. If the trust instrument is silent in this regard, then it would be a majority-rule situation; if the 4th sibling feels that the majority is in some way damaging his/her interests, then he/she could file a legal action seeking a judicial declaration of what can and cannot be allowed, or is or is not proper in terms of disposition of the property. Since such a legal action can be expensive and acrimonious, it's not to be done lightly, but does provide a dissenting 4th sibling a forum.