Tank-ace... do the math. Im not going to spell out the entire process for you. Either devote the time to learn how it is done or just admit there are people that can do things you cant figure out.
Next time you are on a vbase and things start to explode but you cant see incoming rounds you will know what is happening.
If I had just four people that could do this then your first hint of a pending base capture would be a flood of Jeeps. Thats why I dont go around telling people how to do it. It would destroy the game. Thats how I see it.
Practically speaking I think the mountains and hills should be reduced to a maximum of 5k or the distance from any mountain that reaches above 5k be moved 20k from any field or town. But what I see as a danger probably doesnt exist of the great mud puddles cant figure it out. 
The numbers still add up to a LOT of wasted shells on your part. If you can quickly get fire anywhere NEAR your target, you're firing from a pre-ranged possition. If say you can quickly adjust fire onto a specific building, you're a damn liar. If you say you don't run into
any problems with the recoil, its a mirical you haven't choked on the BS thats poring from your mouth.
The M4 can't do what a Wespe with a variant of a land-gunner mode could. It can't quicky, consitantly, and efficiently deliver heavy fire onto a distant target, without hours of preparation.
So, while your M4 might work as indirect artillery out to 4000yds or so, past that you're running into more troubles than the fire provided is worth, and likely insurmountable problems without untold hours of fire and correct.
And I admit you might be able to do it at 15k; we thought you meant direct fire from 15k, as thats what we were discussing when you brought it up. However, you won't be able to do it 1) quickly 2) consitently 3) efficiently
But I think the first sign of a base capture in progress I would see is your scout circling around town for the hour it would take you to destroy all the town buildings, shortly before some 30mm fire stiches across his fuselage. If you tried to jump the gun, I might also see a horde of jeeps swarming towards the town to try and machine gun the town down and try and rescue a poorly-planned capture attempt.
Why? No v-bases are within 8.52 miles of an airfield, so far as I remember, a scout would quickly be shot down if he were orbiting an useable airfield, and if the airfield weren't useable, the horde would already be overhead.
So yeah, you could theoretically provide indirect fire at extended ranges with an M4, but you'll never be able to do it with anywhere near the usefullness of an actual SPG.