Aha! I finally found something! This does not tell me, specifically, how common the rs4702 gene mutation is in schizophrenics, but it does tell me that it seems to be present in about 30 - 50% of the general population!
Woohoo!
rs4702
Current Build 154
Released April 21, 2020
FEEDBACK
Organism
Homo sapiens
Position
chr15:90883330 (GRCh38.p12) Help
Alleles
G>A / G>C
Variation Type
SNV Single Nucleotide Variation
Frequency
G=0.341082 (42829/125568, TOPMED)
G=0.438634 (49520/112896, ALFA Project)
G=0.35591 (11157/31348, GnomAD) (- 16 less)
G=0.3538 (1772/5008, 1000G)
G=0.4004 (1794/4480, Estonian)
G=0.4385 (1690/3854, ALSPAC)
G=0.4539 (1683/3708, TWINSUK)
G=0.4549 (1333/2930, KOREAN)
G=0.4208 (877/2084, HGDP_Stanford)
G=0.3081 (583/1892, HapMap)
G=0.4525 (829/1832, Korea1K)
G=0.416 (415/998, GoNL)
G=0.393 (236/600, NorthernSweden)
G=0.455 (243/534, MGP)
G=0.244 (108/442, SGDP_PRJ)
G=0.463 (100/216, Qatari)
G=0.491 (104/212, Vietnamese)
G=0.27 (13/48, Siberian)
A=0.47 (19/40, GENOME_DK)
Took me long enough.