4 seconds is hyperbole, but certainly under 10. I have the splash-screen turned off, so I can see the counter during the kernel boot messages, and that usually gets to about 4 (sometimes less) when it switches video modes to go into X. It's only about a second before I have a prompt, then logging in to KDE and getting a completely working desktop takes about 2 seconds. But still, all that together is almost less time than the POST screens.
Everyone NEEDS an SSD. The difference in day-to-day use is radical. Imagine being able to get Firefox back in like 2 seconds after it crashes. When I have to use my laptop, which doesn't have an SSD (yet), it's like going from a Ferrari to a tricycle. I used to say that if you wanted to build a fast system, trim the amount you budgeted for a processor and throw in enough RAM to make the motherboard groan under the weight of all the DIMMs. You should still do that, but you should also get an SSD. And I think they should be mandatory for laptops, which are cursed with pokey drives to save power. An SSD will save a lot of power while silently delivering a huge leap in performance. The 80 gig Intel X25-M has dropped under $200 now, which makes it about the best bang/buck way to speed up your system.
It actually makes me kinda angry. All this time computers have been a lot slower than they need to be, all because of these stupid, slow, noisy, power-hungry, mechanical hard drives. After feeling the difference first hand, all I can think is "what the hell have we been doing with these stupid spinning disks all this time?" OK, rant over.
I thought people with Linux rarely booted their systems?
I booted it a lot getting all the hardware set up and adjusted and getting OSes installed. I also had a couple parts arrive later (quieter fans and a controller) so I had to shut it down to install those. Also, I dual-boot into Windows for games (if I ever get time to play them ever again...) So I've gotten a good feel for boot times.

But you're right, unless you're shutting your computer off at night, or you install a new kernel update, it's rare to reboot even a desktop Linux box - up-times of hundreds of days are typical. It's common to have an up-time of years on a server. But I'd expect a Mac to be similar, being based on BSD.