Of course, one realises later how one's overcomplicated things; a normally-entered formula in BA4 seems to work well:
=IFERROR(INDEX($B$1:$AV$1,MATCH("ynnn",INDEX($B4:$ AV4&$C4:$AW4&$D4:$AX4&$E4:$AY4,,),0)),"not found")
Of course, one realises later how one's overcomplicated things; a normally-entered formula in BA4 seems to work well:
=IFERROR(INDEX($B$1:$AV$1,MATCH("ynnn",INDEX($B4:$ AV4&$C4:$AW4&$D4:$AX4&$E4:$AY4,,),0)),"not found")
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