using float4, I get incorrect value when select

Hello, I have a table, with a float4 column, when I insert a new row and for
example I use this value "5000542,5" in the column of type float4, the
insert do OK, but when I select (select myColumn from myTable) I get the
value "5000540" I don't know why.

But if I cast to float8 (select cast(myColumn as float8) from myTable) I get
the correct value ("5000542,5").

I don't know if I am doing something wrong.. Please help me. Thanks

P.D.: I am using postgres 8.1 for windows

Anibal