Thank you!!!…

Requires 2 builders for one tile.

One turn before acquiring knowledge for Level 3 improvement (e.g. Large Precious Stone Mining for Diamonds), move one builder over the top of the Level 2 improvement.
After the technology advance, redeploy the first builder away from the improvement, and send second builder to improve the resource to Level 3 improvement. Then simply cancel the redeploy orders of the first builder – the first builder will appear in the same tile with the second builder who is already improving! Simply give orders to the first builder to improve the resource to Level 3.
In one turn, you will get 2 builders improving the same tile, for the cost of 2* (8 Lumber + 8 Iron) or 16 L + 16 I total, for improvement from Level 2 to Level 4 ! The technology for Level 4 resources is not required at all!

I tried the same with stacking up 2 builders in same tile one each turn) and giving orders to 3 builders simultaneously to improve from level 1 to level 4, it worked also. Yet, improvement of Level 5 can be achieved on the screen info, but you won’t get 5 resources per turn.

I usually improve the tiles to Level 1 without improving to Level 2 (its cheaper to build Level 1). When I can afford it (by allocating 20 Lumber + 20 CastIron in 4 turns), I improve the resource to Level 2 and after 2 turns I am sending extra builder to improve from Level 2 to Level 4. Usually I have the technology for Level 3 improvement when I do this. Otherwise, I should keep the builders on the tile one turn before disvovering the required technology.