1. the beneficiary brother died subsequent to the father
2. so the bequest in favour of the brother is valid
3. as brother died, the bequest to him will then devolve on his legal heirs i.e. his 4 daughters and widow (if any)
4. as in 1983 there was a court decree in favour of younger daughter for 4 marlas of land, the father could make a bequest for only 6 marlas of land
5. so the Will of the father in favour of his son for 10 marlas will be valid only to the extent of 6 marlas, as 4 marlas out of the whole land had already gone to his daughter (i assume the decree in favour of the daughter was not challenged, in which event it becomes absolute)
6. the legal heirs of the brother can prove the Will in favour of their father by filing a petition for letters of administration with Will annexed under which the 6 marlas of land will be transferred to them by execution and registration of transfer deed
7. the other legal heirs of the father are all excluded since under the father's Will, the property is bequeathed to the son. They can however object to the Will by saying that it is forged or fabricated. So the legal heirs of the brother will either have to file LA petition (which will get converted into a regular suit upon receipt of objection) or file a declaratory suit in court to declare them as co-owners of the property
9 your last query - unregistered Will is valid. It just needs to be proved as explained above