How to level up shooters from lower to t4?

Once your fighter/shooter/rider/vehicle Camp has been upgraded to be able to make level T4 units, you can upgrade your T3 units.

Open the Camp and then click on T3 on the level scroll bar (under the large picture).

You will then see a gold up arrow in the upper left corner of the large picture.

Click that, and you can select how many troops to upgrade (limit depends on resources and unit building limit).

Repeat as needed until you’ve got all your troops upgraded to current level!

Hope this helps! Have fun!