The cost of creating your 4th company is 200 concrete. The cost of upgrading Automation Engine to lvl 5 is 160 steel. Steel and concrete are similar in price so for this exercise, we can see them as the same Unit.You might think that since both actions give you 24 production points, then it's more efficient to go for the Automation Engine upgrade.
However, a new company means the first upgrades to the AE are cheap.
Here's another way to look at it: cost of bringing a new company to AE lvl 4 is 200 + 20 + 40 + 80 = 340 U, the cost for an extra 96 production points. Whereas, upgrading 4 companies to AE 5, would mean 160 * 4 = 640 U. Huge difference and a reason to bring a few more companies to AE 4, before upgrading all of them to AE 5.
The cutoff would be the 9th company at AE 4 and a cost of 450+20+40+80=590U, but one should also consider the cost in assigning XP points, a topic for an altogether different and much more complex discussion.