I'd actually say ALL the time. First of all, the trade isn't finalized until the second team gets the guy they are targeting
For example, last year the Hornets sold their late-20s pick to Portland the day before the draft. I also have vague recollections (without any specifics) that trades where a lower pick is thrown in often go down before the lower pick -- so if we were to trade #4 and #23 for #2, the trade would probably not happen until after #4, but could happen before #23.
But yeah, if there's a particular player being targeted, the trade is on hold until that player is actually selected.
and secondly if a trade were announced before the second player was picked, the team above them could draft (or just threaten to draft) who they were targeting and have some leverage over both teams involved.
I agree that it would be very unlikely for a trade to be announced before the #4 pick in that scenario, but if it were it doesn't necessarily give OKC any useful leverage.