TL;DR: This module needs 6 hours, not 4.

I have very mixed feelings about this module. On the one hand I really appreciate how unconventional this module is, a CSI-styled adventure with encounters that requires creative solutions (rather than killing everything, Chiang Emporium anyone?). On the other hand, there is simply too much information for the players to attempt to obtain and digest within the given time frame, and it's almost like the module was set up in a way that "forces" the players to fail in their investigation.

Here's a breakdown of my opinion:

1. Firstly, the connection between the evidence available to a mindflayer behind all this is close to a bare minimum, unless the DM explicitly provide the table with hints. Otherwise, the best they can deduce is that someone is behind all the incidents, mind controlled cranium rats & pixies are involved, and the person-in-charge is a "purple man".
2. Secondly, the only way the players can find out specific information about who the suspect might be is to ask the NPCs specific questions about other NPCs, which I can't say is something people would think of from the top of their heads when questioning the witnesses of any given crime, especially when the players have no known relationships or former interactions with any of the NPCs. A similar module, CCC-LINKS-01 Champion of the People, has a similar issue. However, since that mod only has four major suspects, and all suspects know each other to a certain degree, it makes more sense for the NPCs to relay information about other NPCs naturally. I cannot say the same for this module.
3. Thirdly, forcing the players to make a "wrong" decision isn't fun. Through the hands of the mayor, the players are put under a situation where they have to pin someone down halfway through the adventure, even though the odds of them having sufficient information to deduce who the suspect is is very low (for the aforementioned reasons). Their characters' alignments aside, being explicitly pushy over the players' actions doesn't seem like the best idea, especially when they are likely frustrated over their investigation finding nothing.
4. Fourthly, the "way-out" of this module seems to be just simply interrogate all suspects until the Intimidation check goes through is just..... sub-optimal. Partly because resolving to yelling at your suspects when you can't find any clear evidences doesn't make that much sense in or out of character; partly because in a RP-heavy module like this one, I as a DM prefer to act out all RP interactions, and I simply cannot imagine realistically how I can RP the interrogation.

Furthermore, speaking to all those NPCs take up a lot of time, have the players decide on who to target takes even more, and then there are a potential of two final encounters. In my case, I had to cut both final encounters and settle for a lower xp for my table, simply because I don't have time, which is a shame. With the bulk of information provided in the module, the players and the DM need to be presented a way to efficiently nagivate their way through the investigation phase, or have the bulk of information itself trimmed down.

P.S. Damn that Flametongue weapon is cool. I wish I can get it as a Dedicated DM reward. Ah well. A man can dream.