After finishing a entire playthrough i wish to leave a few comment about researching technology:

1, The UI; So, unavailable to research is greyed out which is fine, but i wish for a better distinction between currently available, currently researching and researched nodes (looking at the color/progress behind the tech name isn't very telling at a glance)

2, It's really easy to screw up by specializing. Since you can only research 1 tech at a time, i often find myself trying to focusing on a single branch, and 10 turns later i realize i was wrong and the tech i need is hidden few nodes behind another branch, and the game becomes very difficult.
I hate to bring up comparison with stellaris, but in stellaris you can research 3 categories at the same time, which let you specialize and progress at the same time, there is no scenario in stellaris where i go "oh crap i forgot to unlock that earlier tech", and in GC4 it's happening over and over, and i have to go back and check which early tech i need to research.

3, i feel that the building unlock is a bit underwhelming. In my last play through, the way i unlock buildings is more like "the building unlock happens to be in the process of unlocking that tech i want few nodes down the line". Which makes building unlock more like a "bonus". If that's how it's intended then fine, but i think is much better if we can unlock different classes of building very early, and just upgrade the building when the tech comes up.
An example: player will unlock ALL the building that gives +1 bonus when the very first tech in a category is researched.
Player will unlock ALL the "multiple +1" buildings with in the first few nodes of each tech tree.
This way, player can actually plan out how the planet will progress, instead of adding new buildings and trying to figure out how they will fit.