The PMBD3640 is the standard option for an LG 4 port brnach box. The PMBD3641 includes a larger port specifically for use with the a 36k BTU indoor unit. Unless you are using one of these indoor units, there is no benefit to using the PMBD3641.
LG includes any necessary port adapters with the air handlers,so they do not need to be purchased separately. They specify specific line set sizes based on the capacity of the indoor unit, and the size of the line set, along with the connections available on the indoor unit and branch box, dictates the location of the adapters. The adapters may introduce a minimal restriction, but this is insignificant to the efficiency or performance of the system. Using a line set size other than the one LG prescribes could result in poor performance.