If you do wish to downgrade, there is an existing thread in the [Only registered and activated users can see links. ], you may have missed it when/if you looked there: [Only registered and activated users can see links. ]. Read it all because there is more than one way to downgrade fw.

Sometimes problems can occur from added files such as an 'unofficial' map or GPI file. Try removing any recently added.

If you do need to reformat the 16GB microSDHC, you must do it in FAT32. Windows has a built in tool for that and there are 3rd party free programs you can google.

I'd be looking at other troubleshooting options first before you downgrade, as i haven't heard any other complaints about V4.00 and it's been around since 11th December 2013. Although i have a 3597 myself, i'm still running V3.80 as it's been stable and problem-free for me.