The Direct3D9 backend isn't something you can just "transfer" over; there are lots of bits of code you'd have to splice together and alter. It was removed in the latest revisions for several reasons.
You can either continue to use 4.0 or turn to unofficial builds such as DX9 Final. Since those builds in that thread are unsupported, and issues you have with them you'll have to ask Tino about it. However, is it necessary to use newer revisions of Dolphin? Is there a specific issue that newer builds solve that is broken in 4.0? If not, consider sticking to 4.0 for the time being (that is to say, until your next hardware upgrade) if nothing is wrong and your gaming speed is fine.
Alternatively, try the very latest builds (4.0-750 at this time of writing). There have been a lot of speed ups to Dolphin, and it might give you fullspeed whereas 4.0-468 doesn't. EDIT - Also note that 4.0-468 doesn't technically exist. Dolphin's master branch builds jump from 4.0-436 to 4.0-476.
You can either continue to use 4.0 or turn to unofficial builds such as DX9 Final. Since those builds in that thread are unsupported, and issues you have with them you'll have to ask Tino about it. However, is it necessary to use newer revisions of Dolphin? Is there a specific issue that newer builds solve that is broken in 4.0? If not, consider sticking to 4.0 for the time being (that is to say, until your next hardware upgrade) if nothing is wrong and your gaming speed is fine.
Alternatively, try the very latest builds (4.0-750 at this time of writing). There have been a lot of speed ups to Dolphin, and it might give you fullspeed whereas 4.0-468 doesn't. EDIT - Also note that 4.0-468 doesn't technically exist. Dolphin's master branch builds jump from 4.0-436 to 4.0-476.