NULL values in where clause

i've got a table "bla" like this:

[id]    [name]    [fk]
1       test      4
2       foo       5
3       bar       NULL

if i do the sql query

SELECT * FROM bla WHERE fk <> 4

i only get the record with the id 2. i don't get the record with id 3 where fk is null. I thought NULL != 4. Seems that this is wrong.

Why is this so?