sorry for the delay, you know, this is not service center, this is just people community. Just people, with their deals, their problems and so on:)
Let metry to answer your question, I think you can use the ippiCopy functions, namely ippiCopy_8u_AC4C3R and ippiCopy_8u_C3AC4R to convert 3 channel images to 4 channel's images and vice versa.
In addition, I recommend you topay attention to IPP manual, which contains a lot of details about IPP functions.
copy 24bit bitmap to 32bit bitmap buffer
Is there any function providedin IPP to copy 24b bitmap to 32bbitmap buffer?