Generally this dollar symbol following a number is used to avoid manual execution of commands.
For example you have a .nc file with different dates (i.e. varying l values), and your coomands in ferret are like this
yes? fill prep[l=$1:$2@ave]
these commands fill your area with intended months average value.
for these commands to execute you have to type
yes? go dummy.jnl 10 20 ; this will average your prep values over 10th to 20th l values and plots.
But as I understood from your query you have given number 4 directly, you have follow a serial 1 2 3 .........
I think this will solve your problem.
Manche shiva shankar