Well, maybe there are simpler ways, but I wrote this, and seems to work well...

RTRIM(char(integer(<DOUBLE_VALUE>))) ||'.'||right('00000'||trim(char(integer(round((abs(<DOUBLE_VALUE>)-trunc(abs(<DOUBLE_VALUE>),0))*10000,0)))),4)

The underlined values might needed to change if 4 decimal digits is not the choice you need...

Take care!
István