You need to decide on a budget and timeline. In other words, how much do you want to spend immediately and how much down the road. This allows you to either implement a full solution immediately or complete it in stages. The latter will typically allow you more room to spend on higher quality / more costly equipment.
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Perhaps the starting point is based on "personality". Seems like people are either "keepers" or "swappers"; this seems to apply to AV gear, cars, cameras, watches, etc. I fall into the keeper category, buy good stuff, keep it and add to it as budget allows. If your are a keeper then start your system with the 4B and add to it.
I started my Bryston adventure a decade + ago with a 3B and then over time added 6B, 7Bs, SP, BDA, BDP, etc. I may have reached my "keeper" end point, however, I just counted 20+ pieces of electronic gear, excluding speakers, in my AV area.
Steve
Steve