ruby driver postgresql

HI
I am using the C driver and am trying to run this statement
irb(main):020:0> a=conn.exec("select * from exchanges where id=$1",4)
ArgumentError: wrong number of arguments (2 for 1)
from (irb):20:in `exec'
from (irb):20
from :0
irb(main):021:0>

I get the error. But the ruby doc stats that i can use this parameteric
query. any pointers will be useful.
Regarfds

Seede