There’s three reasons why I’d say the FP3+ could be a better choice: its slightly smaller form-factor, its up-front expense and its 3,5mm jack without a separate USB-C converter. In all other cases, I think FP4 works out as a better deal.
If you look at the cost-per-year of both phones under reasonable assumptions, I’d say they’re pretty similar. These assumptions are that the FP3+ has 4 years of support, spare parts and software updates left, while the FP4 will still have 5 years of that in the pipeline. That means that the cost-per-year of the FP3+ today works out to €439/4=€110/year, and the FP4 is €579/5=€116/year. In other words, their price-per-year is pretty similar. For the FP4 you get overall better specs (more RAM and storage, better camera, bigger battery). and with its 5G support I’d say it’s more future-proof too. Unless the FP4 is significantly more wasteful to produce or is built to a lower quality than the FP3+ requiring more repairs over its lifetime (which I doubt, but the proof of the pudding is in the eating), a longer life also means less waste.