In Episode 3 we learned that the FISA Court was not told that Carter Page was a long time source for the CIA. That was the biggest blunder in the case. At one point an FBI lawyer altered a CIA email to make it appear he didn't work for the CIA. And that's the focus of Episode #4.
Government leaks to the media portrayed Carter Page as a suspected Russian spy. He claimed he wasn't and said so publicly. That prompted a member of the FBI's Crossfire Hurricane team to request a definitive document to support or deny Page's claim that he worked for the CIA. The FBI already had a CIA email supporting Page's claim. But an FBI attorney doctored it to say that Page was not a source for the CIA.
One of the most important resources to understanding "Spygate" is a report by the Department of Justice Inspector General. It was issued in December, 2019. SpyGate Truth repeatedly refers to it, citing page numbers for the source information we report. You can look for yourself. Here's a link to the full report. Be advised, however, it's 474 pages long and does not read like a spy novel.