I'm in ch 4 and I need to know how to make riku the culprit for the mi..

Well, actually it's hard NOT to make her the culprit!! All you really have to do is go to each area and call rin multiple times. Some commspheres, you have to wait a little while before sumtin happens. Just wiat till sumtin happens, den call Rin. In chapter 5, it should get to the conclusion dat Rikku was the criminal.
NB: You must do the "Catch a Chocobo!" Mission in Chapter 2 (Coz Rikku jumps off the tall structure and lands on a machina, causing them to go haywire!)((Dat's what the footprint is on the bot))

If you got someone else, please tell me what you did, i'd like to know how to get other suspects...

I'm busy with a Mi'ihen Mystery Walkthrough, you can check it later once it's done, i'll give you a shout

First, go to the Ruins and wait a bit... You will see a machina come out from behind that big stone stucture, call Rin and he'll tell you that a footprint is on the machina drone. Then go to the Hover crash sight and see the man thats injured (Call Rin when all the Machina run to the bushes). Then go to the MRR Entrance and See a dodgy man "talkin" to a mini-chocobo eater (call Rin when he starts "talkin"). Then go to the Travel agency (Front) and Call Rin when the Mini-CE chases a common Spira Gull. Then go to the back and Call Rin when an Al Bhed Lady looks at the Screen thingy. (Travel Agency Back). Then go to Shinra's CommSphere and Call Rin. Then go to the Ruins Again and call Rin when the Man Starts talkin to the same Man that was at the MMR and Rin will tell you they seem to be doing a deal of some sort. Then go back to the Hover Crash Site. Call Rin and he'll look behind the Hover and tell you that he found a key piece of info. Go back to the MMR Entrance and see the Al Bhed Lady Run off when you Call Rin when she speaks to the Dodgy Man. Then go to the Last CommSphere and talk to the two gentleman(By calling Rin). Then go to the Travel Agency Front and Call Rin. He'll tell you that he patched up the wounded man and that he found another key piece of information and that he would take over the rest of the case. In the end, Rikku is the Culprit because during the "Chocobo for Calli?" mission, she jumped off the stone structure and hit the machina drone. And at the end, when Rikku calls "Wait Up!", the hover had to swerve to miss an "Al Bhed Lady", but you'll see the scene 4 urself!