The question is not so simple to answer because what you need to achieve with regards to your water, is not so much to get the pH down, but also to get the kH down. If you have the kH of 4 you need to realize that you can try to force the pH down by adding some or other acid, but the pH will bounce back until you have actually completely neutralized all the carbonate hardness.
What is important when you keep altums is to try to get the water as soft as possible and not only to get the pH down. If you use acid to get the pH down or if you use discus buffer to get the pH down, you are adding more dissolved substances to the water, so effectively you are hardening the water, which you do not want to do if you are keeping altums.
So you should try to remove the carbonate hardness and then you will see that you pH will drop in any case. What I would advise is that you filter your water over peat, this will remove the carbonate hardness and reduce the pH at the same time and will also add tannins which will be very good to your altums.
We have discussed this before extensively on this forum and you can read more on viewtopic.php?t=721&postdays=0&postorde ... s&start=15
We look forward to hearing about how your altums will settle in once you get them.