So you finally made it to T4 -

What’s the first thing you upgrade?
What radical changes to your settlement have you been just itching for?
Do you have a particular order in which your upgrades are scheduled?
Do you go for heavy weapons and plate mail? Build up your barns? Erect that fort? Or do you increase your entertainment and desirability of your village?
You’ve spent hours getting this far, where will you go next?

With 0.8.3, the first thing is to go over all the mines and see which ones can be replaced/augmented by Deep Mines that I have already noted on the map but couldn’t access until T4.

For instance, on current map just gotten to T3, but have already marked for Massive Exploitation a Deep Clay Pit/Mine right at the edge of the settlement which will end all my Clay resource worries forever (right now, only Clay Pit is 2/3 of the way across the map requiring strings of carts going back and forth to access the clay, a royal pain in the maximus)
But usually, one of the first things to do at T4 is to try to make all the production chains more efficient - look at travel times for the various production buildings, placement of storage facilities, stocking of markets, etc. and move or build new until I’ve got everything efficient - it makes a HUGE difference in making the final push to 1000 population.