He means use vmware instead of HDD4, then you can limit the effect of any bugs and virii. This doesn't solve your original question but it is something to think about.
Have you actually tried booting with the other drives connected but without power? I would check each OS install boots with the other 3 drives not powered, if they all work then go buy a triple pole 4 throw switch.
Yes, I understand what he means. I could install that on HDD4 anyhow. But as you said it doesn't solve my original question. Your suggestion of trying all but 1 HDDs with their power connection disconnected, but I don't have all 4 HDDs. I only have 1 HDD, but planning to buy more HDDs. This thread is to find out whether it'll work with 3 HDDs off and 1 HDD on at boot-up time. If so, I would then buy more HDDs, knowing that someone out there has done it and had no problems.