I also e-mailed eureka and got an identical reply... especially as series 4 and 5 are probably the most popular series' they stand to make more profit on releasing the rest than on the first three. I also e-mailed the BBC and they said:
Thank you for your e-mail regarding '2 Point 4 Children'. Whilst I appreciate that you are interested in obtaining a copy of series 4 of the programme on DVD, I must advise you that I am currently unaware of any plans to release this series. However, if you would like to make a suggestion for its release, I would suggest you put it in writing to the below address who are responsible for the commercial release of all BBC DVD's. Please write to: Programming & Acquisitions Dept2 entertain33 Foley StreetLondonW1W 7TL Please note that there must be adequate public interest before a release can be arranged.