I was about to ask if the investments made in the provinces would be carried over, but read that they wouldn't due to problems with sensibly exporting them to EU-IV. At least not per se.
However this got me thinking... maybe they're abstracted to the province base tax value, manpower and fort level! The CK2 buildings are either fort level, money makers, troop increasers or tech levelers...
Fortification level (either average or the highest in the county) makes the fort level go up, so you have a max fort level in CK2 as Level 2 fort in EU IV, and basically all the rest as Level 1 forts.
The tax value of the province, without modifiers (recently conquered, different culture, etc) would translate to some number of base tax value, while other conditions are transported separately (mainly culture, religion and non-core in EU, I think?) which in that engine affect the base tax of the new province.
Troop increasers would affect manpower similarly; the more troops there are in a province in CK2 - as a raw number, not just the ones you can raise but the maximum raisable - would determine the base manpower of the province.
Tech level... I imagine would be determined by the tech level of your capital, and as there are also three "trees" in CK2 and EU4, military, economic and social vs. military, administrative and diplomatic... then the total number of techs in each tree get abstracted to the general tech level in the nation? So an 867 conversion should have 1444 Europe at tech level 1 or 0 almost across the aboard, while a later on game might have them at 4/4/4, or even a maximum of 5 or 6.
Starting standing armies... I assume would be based on retinues? No idea on how the starting navies might be converted.