I agree with Phil... unfortunately, the home theater market IS like PCs... there's newer, faster, more features every few years.
To Bryston's credit though, they're keeping the SP3 up to date. It was originally produced with HDMI 1.4 pass-through. Then they offered HDMI 2.0 as an upgrade and on newer units. Soon they'll have HDMI 2.0 with HDCP 2.2. Aside from Atmos decoding, this is allowing me to keep my SP3 and not feel like I'm missing out on anything.
Is DTS:X (and Qualcomm's version) out yet? If you're getting a new receiver and Atmos is important for you, I'd make sure it's up-gradable.
I suspect Bryston will wait to see if Atmos, DTS:X, and any other formats catch on and become relevant before investing in a SP4.
- Garrett