This may have more eyes if it had been posted in the Hand Tools forum.
I recently sharpened the blade and lapped the sole for a friend's Stanley No. 4 which was originally purchased by his father.
The father likely used this for his woodworking hobby.
After a slight lapping I could see the sole was concave side to side. This may have been from the factory, but was more likely wear over the decades from planing edges of boards.
As Bonka mentioned, the area around the mouth is the most important.
I lapped until the area in front of and behind the mouth were level. This ended up with most of the toe being level and the outside edges being level.
If the area around the mouth is not level, the plane will work, but the shavings will not be consistent thickness across the shaving width.