If I understand the situation correctly, legal title of the property was in the names of H & W only, with the beneficial interests set out in the deed of trust/deed of gift. On this basis, H & W were the trustees of the trust of land created under the deed, by which the current interests were defined.
If this is the case, and in the absence of any mechanism within the deed specifying who can appoint new trustees of the trust of land, the executors of the last to die of H & W have the power to appoint trustees (s.18 Trustee Act 1925).
Whilst the fact a person has learning difficulties does not mean that he cannot be a trustee of the trust of land, if he were a trustee might that be detrimental to the administration of the trust, especially if there were concerns that he would not sufficiently understand what was happening at any particular time, which could impugn any action the trustees needed to undertake.
Is there a deputy (or attorney) in place for the fourth son? If so, then it is their place to protect their principal’s position and the client executors should be encouraged to discuss the situation with them, and for them to be independently advised (if they consider it appropriate).
If the fourth son has no representation, should he have?
If you have doubts that your clients will uphold the deed of trust/deed of gift (as would appear to be implied in making the posting), you might want to meet with the fourth son, or his representative, to satisfy yourself that he is amenable to the proposal. However, this would potentially undermine the relationship with your clients and create a new duty to the fourth son even though he is not your client.
However, if there is no evidence that the executors might be looking to act against the deed of trust/deed of gift, I believe you are bound to take their instructions at face value and look to record on the title that the trustees hold the land for the 4 sons beneficially, also referencing the deed of trust/deed of gift.