If I remember right, they do not interchange.
You are correct, the No. 1 magazines do not fit the No. 4 as is. You will encounter No. 1 rifle magazines that have been retrofitted to work in the No. 4.
That is not the major cost issue that it was as both the No. 1 and the No. 4 repro magazines are now available for a reasonable price that are remarkably similar to the originals and they seem to feed well. Both Sarco and Numrich are stocking these replacement magazines. These are not the ProMag replacement ones which can work but do not resemble the originals.
Restoring a Savage No. 4 can work, given that it is already D&T'ed, if the chamber and bore is okay, then you might want to fix it up as a No. 4, Mk. 1 T (for sniper rifle). In reality, few Savages were converted into T rifles for whatever reason but the duplicate sniper scope mounts are available. You can also find scope mounts for the No. 4 that are not original but if you put a period scope on it can look close to the original look of the No. 4 Sniper.
You can also fix the d&t holes with blind plug screws and given that the No. 4's were usually painted in Suncorite, there are pretty close substitutes to that in spray enamel paints for engines and the like which is far easier than matching bluing.
I recently restored a Longbranch No. 4, Mk 1/3 and the cost was not that bad despite starting with only a barrelled receiver. Finally obtained a Lithgow No. 1 WWII era receiver that I am restoring as a wall hanger. The No. 1's right now are a bit more expensive to restore than the No. 4's. due to the shortage of forends and the delicate front handguard. Buttstocks are available for both rifles pretty cheap and replacement forestocks for the No. 4 are available even in New Old Stock condition. Handguards are also available as is the missing barrel bands, etc.
BTW, the British issued buttstocks in bantam, short, regular, and long.
"Stock, Gun, Shoulder, Bantam.Walnut, 31.24 cm (12.300 in) lg, 12.14 cm (4.780 in).
Stock, Gun, Shoulder, Short. Walnut, 35.51 cm (12.800 in), lg 12.14 cm (4.780 in)
Stock, Gun, Shoulder, Normal. Walnut, 33.78 cm (13.300 in), lg 12.14 cm (4.780 in)
Stock, Gun, Shoulder. Long. Walnut 35.03 cm (13.800 in), lg 12.14 cm (4.780 in)"
https://forums.gunboards.com/showthread.php?395622-No4-Mk2-Buttstock-Length