Quick take
For four people, 48K is often the safer mainstream starting point. Moderate hardness may fit 32K; very hard water or high usage can justify 64K.
Example at 8 gpg
At roughly 240–300 gallons per day and 8 gpg, daily hardness load is about 1,920–2,400 grains. A 32K-class unit may be perfectly adequate.
Example at 15 gpg
At 15 gpg, daily load rises to roughly 3,600–4,500 grains. A 48K-class system gives more practical regeneration margin.
Example at 25 gpg
At 25 gpg, daily load can reach roughly 6,000–7,500 grains. That can make a 64K-class system attractive if the goal is to avoid very frequent regeneration.
Check peak flow too
Capacity is only one sizing dimension. A family using multiple showers, laundry and dishwashing simultaneously also needs enough service flow to avoid pressure complaints.