Answer: OPQR

AnswerCrossword Clue
OPQRQuartet before S
OPQRAlphabet sequence
OPQRN followers
OPQRN-S bridge
OPQRN-S go-between
OPQRAlphabetic quartet
OPQRN-S links
OPQRN-S connection
OPQROrthography quartet
OPQRFrom N to S