As Trevor pointed out, this feature has been requested several times already. http://lists.gnu.org/archive/html/lilypond-devel/2010-11/msg00405.html

It would be good to have an optional setting, off by default, which causes the repeat count in a volta repeat to be printed above the repeat bar line when no alternatives are present. For example,

\voltaCountOn \repeat volta 4 { ... } % no alternatives follow

would print "x4" above the bar line closing the repeat.

James suggested another way of printing this: """ I've never seen it done like that but seem to remember a normal volta bracket on the last measure of the repeated section with '1-4' on it. "

I've seen both, but the "x4" is indeed commonly used in repetitive music (I remember seeing it in Reich's Vermont Counterpoint original edition).

I'm looking at the score to Reich's "Music for 18 Musicians", and the "x" is always after the number, i.e. "4x". This is how I've always seen it elsewhere. I don't ever recall seeing "x4". If x="times", it makes sense to me to play the music four times, not times four, so I'd prefer the default voltaCount to have the number first, when it's on.

Also, be sure to use the UTF MULTIPLICATION SIGN × (U+00D7), not the letter x (U+0078).

A variant (as used e.g. in Wagner's Parsifal' in the firstVerwandlung') is to write repeat 3x' instead of4x'. This should be written at the end of the volta block.