The problem you describe was/is an issue with v0.64 which was very briefly available back on February 24th. Shortly after releasing v0.64 I received a report of corrupted posts similar to yours. I quickly reverted the released version to v0.63 which had been available for a couple months.
On February 26th I posted a v0.65 beta release which fixed the issues reported with v0.64. If you look through my posts and the comments there was a lot of testing done with v0.65 because of what happened with v0.64.
The downside of the v0.65 release was it resulted in a plugin conflict with WordPress SEO. I have since fixed conflict with WordPress SEO in a beta release for v0.66 (which appears to be solid so I plan to release it) and had the fix confirmed by 3-4 different people.
Is there any chance you installed v0.64 or one of the v0.64 beta releases from somewhere other than the WordPress plugin repository? Had this happened back in late February I could see how you might have gotten v0.64 but after two months, I am not sure where you would have gotten it from.
As for how to fix it, I took a quick look for a bulk post revision revert plugin but didn’t find anything. There may be something available that I didn’t see but nothing jumped out at me. The bulk post editing feature doesn’t offer any solution either.
If you’re an SQL guru you could likely craft a query which deletes content from the post table which where the content matches a regular expression created to catch just the post revisions which contain the wpgform shortcode. That is probably the route I’d go down as reverting 4000 posts will be a lot of work. If you do this make sure you save a copy of your database just in case you need to refine your delete query.
Do you have a recent database backup? That may be an option as well.