Very little different and the cost is as explained a number of upgrades not just 5G
It’s also a new phone and the FP3+ has been around for 2 years.
All your questions can be answered by reading multiple posts.
Repairability, well the USB port is separate etc. all this can be discovered again by reading posts.
You are really asking vague questions, I suggest you read more
Sustainability, what are you wanting to sustain? Sustainability is not a human/global/universal trend, whatever is sustained is always at the expense of something else. As the main issue seems to be cost is that what you are really concerned about? > How about the 5 year warranty.
Fair-trade and fair wages sustain other people in their ability to consume, which in turn puts more pressure on other resources.
The FP4 is a more technologically advanced phone, which you seem to know. FP4 is a small company and developing phones is proportionally more expensive process.
All the above has been mentioned multiple times, so it would seem either you haven’t done your own work at reading the topics or you haven’t really explained what it is you want.
You had better see the official videos, this is only a user forum, with opinions.
The original FP3 was released in 2019 while the FP4 got released in 2021.
So naturally the newer one will get you a longer support window / spare parts availability / software updates etc.
Price differences come down to better components mostly, 5G is only one part of that. You might not want it now, but it will ensure a longer possible life for your phone. I had to give up my previous (still working) phone because of the 3G sunset…
I’m going to add the FP4 has 14 materials sourced fairly up from 10 for the FP3+ (or at least, that is their goal according to their last impact report). The display connects through a ribbon cable which makes it more sturdy and resistant to shocks and display connection issues, but a little more difficult to open. It is also is more modular, containing 5 modules (because USB-C plug became independant) instead of 4. It has an IP54 certification.
This shows Fairphone improve the sustainibility and repairability with each new model.
As said already, I think mainly because FP intended the FP4 as a higher end phone than the FP3.