$x += 4;
or
$x->add_ui_gmp(4);

##</code><code>##

$x = $orig;
$x += 4; # $orig remains unchanged
         # $x has been incremented by 4

##</code><code>##

$x = $orig;
$x->add_ui_gmp(4); # Both $x and $orig have
                   # been incremented by 4