Mail about typecast

I want to do a following operation on the timespan

select overdue * date_part('epoch',usage)::float4 from indusevar;

where overdue is of type float4 and i want to typecast
date_part('epoch',usage) to float4 since dat_part(text,timespan) returns
a float8 type.
Whenever I try to typecast it gives an error 'function dtof(float8)
does not exist'. Can anyone help me to solve the problem.
Thanks in advance for your valuable advice.
Waiting for a response.

Regards,
Vicky