First thing, I am a big fan of Bryston products as I have several components....
When considering the SP4, I was surprised that we have lost features that we have on the SP3 as Pre/Pro unit. If you compare it other Pre/Pro units from Yamaha / Marantz / Integra, it is lacking a more modern features like the DAC, interface, front mounted ports (USB/HDMI/Headphone jack), analogy inputs for two channel audio, streaming capabilities, AES/EBU inputs, and it costs at least 4 times the cost. Your idea to use SP4 + BP-17 for two channel audio is not a practical or a cost effective solution and there is a sonic penalty when sampling rates within this unit are only 48 kHz.
There are no specifications on the performance (SNR/THD/Frequency Response/etc..) of the SP4 from what I see on the website, will this be updated at some point?
From what I see you need a smart phone or tablet to navigate the menus options, no sure if this unit can be fully controlled from the front panel?
I still like the fact that I can quickly push buttons on the SP3 and see the response from the Pre/Pro unit and not worry about the battery level in the tablet / smart phone or where it was last located.
It appears to be more suited for a server based environment than in a listening room, especially with its size - larger than most Pre/Pro units and a fan - noise is a concern.
I realize that updating the SP3 was an expensive option for Bryston , but it has so much going for it