My JM4 was crashing/restarting anytime I tried to save a project to the SD card. I called customer service and learned that SD cards larger than 2 gigs will cause this to happen. I had an 8 gig card in my unit.
So I removed the 8 gig card and the issues was resolved. Reason? For some reason (sorry I didn't get any details from the Line 6 rep I spoke with) when the JM4 was released the largest SD cards out there were only 2 gigs. So in my case when it tried to access an 8 gig card it simply didn't know what to do, freaked out, and executed a re-boot. I can only can wonder what it would have done had I installed a 32 gig card!.
If the moderator of this forum has a more definitive answer to this issue, I'm sure we'd all appreciate learning more.
The JM4 is really a unique product. It may technically be "old" these days, but it is still a very powerful creative tool.