Oh, I suppose I shall dispense with the .... long time lurker of posts, first time posting and all that, from me an all around classic adventure game fan person here with his first post...
SQ4 isn't that rare when it is the 256 Colour talkie Cd version, the 3.5 floppy versions are a bit more, the 5.25 floppy versions are the rarest. Especially the 16 colour 5.25 version.
However the rarest of of the SQ games is SQ1 black box original, 5.25 classic... If you ever see one of these, well I'd say get it, but I am after one myself, the cheapest I've ever seen one on ebay was $500.00
Here are some screen shots for what to look for...