We'll probably be best served doing both:
Acquire a 5th starter now and make a deadline trade, assuming we are in the race. Usually, the best deals available, in terms of not having to send a nice package back in return, are the ones that involve modest to highly paid pitchers, where the team is looking to dump salary.
A JBJ for a SP'er at the deadline seems less likely, since salary dump teams that are out of it won't want JBJ. I guess we could do two deals or a 3-way trade. To me, it might make best sense, assuming there is a trade partner at this late point in the spring, willing to trade us a SP'er for JBJ plus someone else (Chavis? Dalbec? Walden?) or again find a 3 way deal. We may have to pay part of JBJ's salary and pick up a pitcher with a salary that is more than JBJ's. We will need to leave enough budget space for a deadline deal that keeps us under the tax.
It could be a fine line to walk, financially, but without trading JBJ (or DFA'ing him early in the season), I think the budget space left will be too small to allow us to get higher impact pitchers.