First off, You can't contain ALL the dust from a TS. Air flow is the key issue,
You have to have enough air flowing past the blade and all its wonderment, to carry the dust to the suction tube be it 4" or 6" $" is plenty to carry enough dust to fulfill your needs. Back in our school shops we had giant DC and built in system to handle whatever we threw at it. the Planer had a 8" duct, the joiner had a 6" duct and the saws had 4" ducts. volumes of air passed through the system, the key was the air movement keep the air moving and it will carry the dust. With a small system as most of us have (I have a 2 hp DC) the volume of air is limited, so you have to contain the flow. 4" should be enough. Often we constrict the air flow trying to direct where it consentrates. A zero clearance Throat plate will constrict air flow and cause dust to carry through and expell on top and out of the reach of the DC. a wider throat will allow air to flow past the blade and carry the dust to the collection place.
Each has its advantage, the ZC throat plate makes for great cuts but poor DC. Wider throats have greater DC but splinters on the cut. It is a write off as to which you prefer.
You can't make an industrial DC out of a home model increasing the pipingwill only agrovate you and cause an undue expence, Manufacturer provided a 4" duct , so use it. Yes the DC has a 8" hole but it devides into two 4" ducts... 4" is the answer.