From a quick web search, I think Shinobi can use the intel GPU (but you should confirm this for yourself as I don't use it).

If so, I would not buy that Dell Optiplex because it's a 3rd generation processor. (You can tell from the first digit of the 4 digit number following the processor type, i.e. the '3' in i7-3770.) The GPUs in newer processor generations are much more powerful. If you're using software that can use the GPU, you don't need to spend for an i7 ... an i5 should be more than fine.

A 6th or 7th generation i5 is probably the sweet spot in terms of price and should be able to support your cameras without problems. The 8th generation system you are putting together should also work fine, but probably costs a bit more than you need to spend. However, in case you might consider using Blue Iris, there were some reports of Blue Iris having memory leak problems on 8th gen processors when using the GPU -- I'm not aware of whether those have been resolved or not.

Also, do you really need a managed switch, and is it the easiest thing for you to run all the wires back to a single switch? If you don't care about having a managed switch, you might find it cheaper and more flexible to but 4 switches with 4 PoE ports each ... something like this one.

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