Ag lime does not work all at once . It takes effect over time in a curve. For example it may go from 17 to 25 over 4 mths and stay there for a year and start a slow fall over 2 years. The amount you put in determines not only how high it goes but also how long it stays up. With 4 tons vs. 2 tons it may stay above 20 for an extra year.

One way to use lime is to put it in series like 2 tons now and another ton in a year and then add as needed to keep it above 20.

Do you intend to use a fertilizer program if needed. You may not need one and if not then excess lime is not as important.