While I agree with your recommendation about usage of - in column headings, your code fragment is incorrect and misses the closing ), ]:
df[order(-df[4, ]
will order the columns descending by the values in row 4 (if you add ), ] to the statement).
order() returns a list of row numbers that would sort df but you need to also specify the columns:
df.sort <- df[order(-df[ , 4]), ]
df[row, column] so order(-df[ , 4]) sorts rows in df descending by column 4. Adding another , ] will pick all columns in the final result.