There is no way to bring rank 1 Guardians from level 1 to level 15 with an average of 4 hits. It is possible to do it (even with 3 hits) using hits on already wounded warlords but that is not a realistic average since there are not enough higher level Guardians to "prepare" a sufficient number of wounded warlords to upgrade half of all the rank 1 Guardians you need.
To make a rank 4 Guardian you need 4 rank 3 - one of them at level 25
To make the 4 rank 3 Guardians you need 12 rank 2 with 4 of them at level 20
To make 12 rank 2 Guardians you need 24 rank 1 guardians - half of them at level 15
The combined task that needs to be accomplished is to upgrade
12 rank 1 from 1 to 15
6 rank 2 from 1 to 20
1 rank 3 from 1 to 25
How many total warlord hits does it take without any "outside" help meaning that no other Guardians except these 19 are used?
I say it takes around 200 Warlord battles to do it.