Getting Started

According to things Bill has said in the videos I've seen, buying now gets you the Mk4 "base model" with the Mk3 firmware. Once the Mk4 firmware is released, most likely some time in May, you will just need to connect the main board to your PC and download the firmware to it. So as far as that goes, the only reason to wait is to avoid the minimal hassle of updating the firmware. (keep in mind that the new firmware can do fancy things like detect where the double Y is and where the extruder is, meaning you would have less programming such as length of the tubes/distance of retraction, and there was some mention of better Klipper functionality).

As for the "Pro model", that doesn't actually exist. The "Mk4 Pro" is a series of add-ons to the base Mk4, and they will not be selling them. Instead, they will have a STL file to download, and a BOM (Bill of Materials) detailing what extra board/motor/etc to buy, as well as instructions on how to install it. You will print the STL yourself, and buy the extra hardware yourself, and assemble it yourself. So again, there's no reason to wait to buy the 3D Chameleon because you'll have to do this all yourself anyway.

Ironically, the most expensive "expansion" part seems to be the I2C board, which is required if you're going to have more than 1 expansion part because it will allow all the other expansion parts to be plugged into it, and even that's only going to cost around $10-$15. Getting literally everything, despite each individual piece being optional, would still cost less than $40 extra (and that's supposedly including all the bits and pieces to create a "dry box" you probably don't really need that can hold 4 filaments, with buffers built in to "catch" the extra filament that gets rewound out of the machine during a normal switch since the spools don't rewind themselves), plus potentially the filament to print the parts but if you have enough filament to need the 3D Chameleon you probably already have enough filament to print the parts/mounts.