To multiply by four.
To increase by a factor of four.
To provide four parallel running lines on a given stretch of railway.
A figure-skating jump with four revolutions in the air.
Something that is four times the usual number, amount, size, etc.
Being four times as long, as big or as many of something.
To make quadruplex.
Of or relating to an early videotape format with four magnetic record/reproduce heads mounted on a headwheel spinning transversely across the tape.
Having four components.
Of or relating to a system in telegraphy by which four messages (two in each direction) can be sent on one wire simultaneously.
A quadruplex system.