The intercoastal are a series of muscles that are interlaced with the ribs and help provide stability to the rib cage. Straining this muscle is usually the result of a quick and sudden twisting of the core. Matusz may have experienced this during his reaction to being hit with a line drive this past week in his final tuneup against LJ Hoes. The batted ball hitting his biceps would not have been the issue, it would have been him reacting and twisting away from the ball.
Based on the three to four week number, this sounds like a Grade II strain of the intercostal muscles. This is a bad, but not awful strain. It would not be surprising if it takes about a month and a half for him to heal up and another rehab start or two. Right now, he is probably feeling some discomfort while breathing and constant, dull ache in his rib cage.