If you inject once, you have contaminated the needle. You'll likely be fine, but why take the risk? Transfer the solution to 4 insulin syringes, costing you about 60 cents more.
Yeah, strictly speaking, you're right. But take it from me (I'm an M.D.) a used needle is unlikely to be contaminated if you've prepped your skin properly with alcohol & use sterile technique with giving the injection; and more to the point, an infection from a subQ stick isn't nearly as big a deal as it is with IM. A contaminated IM injection would probably lead to an abcess deep in the muscle that would require surgical drainage - but with subQ even if you did happen to get an infection, your body would probably just get rid of it by itself within a few days, just like if you had a small cut that got infected. Sure, there's a tiny risk that it some infectious agent could get into your bloodstream and kill you or something, but that could happen even with a fresh needle if you have't sterilized your skin.
Anyway, I've been doing the multi-dose thing I described for years without ever getting infected.
So while I definitely agree that you shouldn't ever reuse a needle for an IM injection, I wouldn't worry about it with subQ. And of course, NEVER use a needle someone else has used.