Slice 3 of the peaches into eighths and add them to the bowl of a blender along with the schnapps, lemon juice and salt. Blend until peaches are pureed. Set aside.
Dice the remaining peach into 1/4"-1/2" pieces.
In a medium mixing bowl, beat together the eggs and sugar until thickened and pale yellow. Add the flour and beat until combined. Set aside.
Bring the milk to a simmer in a heavy medium saucepan. Using a hand mixer, slowly beat the hot milk into the egg mixture. Pour back into the saucepan and heat over medium heat, stirring constantly with a whisk or wooden spoon until custard thickens. Be careful not to boil the custard or the eggs will scramble. Strain the custard through a sieve into a large, clean bowl. Let the custard cool slightly, then stir in the peach puree and cream. Cover and refrigerate until well chilled - at least 2 hours.
Stir the chilled custard, then transfer it to your ice cream maker and churn according to the manufacturers instructions. When ice cream is almost ready, add the peaches and nilla wafers and continue to mix until well incorporated. When ice cream is ready, it will be like soft-serve. For a firmer consistency, transfer the ice cream to a freezer-safe container and chill for 2 hours.