Easiest is most likely to use list of columns to remove with Delete Columns
Names Default To Here(1);
dt = Open("$SAMPLE_DATA/Semiconductor Capability.jmp");
col_names = dt << Get Column Names("String");
Remove From(col_names, 1, 4);
wait(2); // for demo purposes
dt << Delete Columns(col_names);
If you want to use For loop, loop from the end to avoid issues with indices changing.
-Jarmo