Form 4 is a change of ownership form filed by reporting persons, who are commonly directors, officers and 10% shareholders (each of them, also known as "affiliates"). It is commonly confused with Form 3, which the ownership form filed by someone who becomes a reporting person. An employee is not necessarily a reporting person.
An "insider" is generally just someone possessed of material non-public information ("MNI"). The class of persons includes affiliates, but can also include employees and consultants if they have knowledge of MNI that was disclosed to them confidentially, either express or implied. So, insiders would also include the lawyers and accountants of an issuer, in addition to its employees and affiliates.
To illustrate, as I think everyone understands, a director/officer of an issuer cannot buy/sell stock on the basis of MNI. If that director informs an independent contractor of MNI, things get complicated quickly. If the contractor is informed of the MNI under in the context of confidentiality, then he/she also becomes an insider. If the contractor is informed of the MNI, but has no reason to believe that the information is confidential, then he/she is not an insider and may trade on the information. If the director intended for the contractor to trade on the information, then the director is liable for "tipping," which is a form of insider trading. If the director had no such intention, and the company learns that the contractor is trading on the MNI, then the company must disclose the MNI immediately. The foregoing is an over-generalization of insider trading, but that's essentially how it works.
If a consultant only trades on publicly available information, then he hasn't engaged in insider trading.
I don't know the facts here, but I know that lay people commonly get the ownership forms wrong. I think it's very possible that this guy just made a mistake in referencing the Form 4, because unless he's a 10% holder, there's no reason for him to file (since he's not a director or officer). I took it to be someone who really meant Form 3, and was trying to make a joke about the amount of stock he just acquired. But then, I didn't give it much thought one way or the other, because it just seemed like a silly thing to say (not entirely surprising on Twitter).
As for talk about manipulation, buying or selling stock isn't manipulation unless it's being done to affect the share price. Dumping massive amounts of stock in a short intended to drive down the price and induce a panic, is manipulation. Slamming the stock in order to drive down the stock price, is manipulation. Publishing incorrect information in order to induce buying, is manipulation. One guy buying a big block of stock isn't manipulation, it's buying.
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