With Spinerock Knoll, when it enters, hideaway activates, however, it doesn't specify whether I put the cards on the bottom of my library in any order, or in a random order. Which one is it? This is relevant for Grenzo, Dungeon Warden EDH.
Yes, you choose the order:
702.74a Hideaway represents a static ability and a triggered ability. “Hideaway” means “This permanent enters the battlefield tapped” and “When this permanent enters the battlefield, look at the top four cards of your library. Exile one of them face down and put the rest on the bottom of your library in any order. The exiled card gains ‘Any player who has controlled the permanent that exiled this card may look at this card in the exile zone.’”
Note that the italics text written on Spinerock Knoll is just reminder text, it is not the actual rules for Hideaway.
207.2. The text box may also contain italicized text that has no game function.
207.2a Reminder text is italicized text within parentheses that summarizes a rule that applies to that card. It usually appears on the same line as the ability it’s relevant to, but it may appear on its own line if it applies to an aspect of the card other than an ability.
So that text is simply a summary of what Hideaway means, but it is not intended to be an actual rule you follow. The full rule for Hideaway is what is defined in 702.74.
Yes, you may choose the order:
401.4. If an effect puts two or more cards in a specific position in a library at the same time, the owner of those cards may arrange them in any order. That library’s owner doesn’t reveal the order in which the cards go into the library.