While I've never loaded for a 4 bore, my standard rule is for a ball no more than .005" under bore size. This is for balls up to about 14 bore in size. Over that, if using a tight (thich) patch, one can go as much as .015" under bore size. for the monstrous 4 bore, one might even expand that to .020" - depending on groove depth. That means you should use a ball not less than .995" in diameter. i would go with an even inch and a .025" to .030" denim patch.

Measure the groove diameter to find the depth of rifling. Subtract the ball's diameter from the groove diameter, 1/2 the product and that is the THINNEST patch would can use with a pure lead ball. If the rifling is only .010" or so, ie: shallow, you can get by with a ball cast of straight WW metal - cheap for the asking at any tire changing gas station - usually.

If you meant the groove (not the bore) diameter was 1.015", that puts the actual bore somewhat tighter & the ball will have to be somewhat smaller than mentioned above - by .005" to .020" les than actual bore size. You must know both dimensions to make an educated guess at what to get for mould and patch thickness.

Again, make the ball from .005" to .020" smaller than the actual bore (tops of the lands) and go with 12 to 14 pound denim for the patch.(.023" to .030"). The patch must have some thickness to it, to obsorb enough lube to keep the fouling soft and allow repeated shots. If this is your first ML rifle, you've a long learning curve ahead. Enjoy.