Ok I am using D1.1 for this and I doubt it is much different from D1.5.

The joint TC-P4-S (SAW) has a root face of 1/4" in (per D1.1). The as detailed tolerance is +U,-0 (which means it must be detailed with a 1/4" root face as a minimum). The as fit-up tolerance is +/- 1/16" (which means as fit-up you must have at least a 3/16" root face at a minimum). If the joint is detailed with a root face less than 1/4" the joint designation TC-P4-S cannot be used.

In D1.1 Figure 3.3 Joint TC-P4-S has notes 2,6,7,10, and 11 pretaining to it. Note 6 states "Minimum weld size(E) as shown in Table 3.4. S as specified on drawings. Table 3.4 states on a 6" or thicker plate you must have at a minimum a 5/8 weld.Put a 3/4" bevel on a 6" plate to get a 5/8 weld would give you a root face of 5-1/4"(the +U means the root face gets larger not smaller).
Note 2 states "Joint shall be welded from one side only. Which means if you have a root face that is zero you will theoretically be welding both sides of the joint so it will not be prequalified as a partial pen weld (it would be a full pen weld)