Enable Auto Suspend?

if you have nvidia, then chances that both S3 and S4 will work for you are quite minimal (assuming installed nvidia driver). Mostly this is a bug in nvidia ACPI, but also some difficulties caused by the fact that ACPI is designed for Windows in mind, so linux devs have a lot of problems trying to get ACPI working on every possible variation that work with windows (manufacturer pre-installed windows that understands specific ACPI hacks)

I would go with microchip explanation of S3 and S4 as there is a reason for both suspend-to-disk and suspend-to-RAM.

AFAIK it works for most people i have never had any problems on my laptop or my main machine.

/Geoff
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my laptop has a strange (problay apic) problem with suspending, not worth fixing cause i don't use it.