To make crust: In a deep bowl with a mixer on high speed, whip the 4 egg whites with 1/4 tsp. cream of tarter and 1/8 tsp salt until foamy. Continue to beat at high speed while gradually adding 1 cup granulated sugar, 1 tablespoon at a time. Beat until whites hold stiff peaks.
Butter a 10 inch pie pan and dust with flour. Spread meringue over bottom and up sides of pan to create a pie shell.
Bake in a 275º oven until crust is pale brown and feels dry to touch. About 1 1/2 hours. Cool.
To Make filling: in a 1 1/2 to 2 quart pan mix gelatin and 1/4 Cup granulated sugar. Stir in water and let mixture stand 5 minutes. Add egg yolks and whisk to mix well, then stir in milk, eggnog, and 1/8 tsp salt. Stir over medium-low heat until mixture is thick enough to coat a metal spoon in a thin even layer. 6-8 minutes. Cover and chill until mixture is thick enough to mound slightly when dropped from spoon.
In a deep bowl with a mixer on high speed, whip 2 remaining egg whites until foamy. Continue to beat on high speed and gradually add the last 1/4 Cup granulated sugar, then beat until whites just hold short stiff peaks. Gently fold eggnog mixture into whites.
Assemble pie: Pour eggnog filling into meringue crust. Cover and chill until filling is set, at least 1 1/2 hours.
Whip cream with powdered sugar until it holds soft peaks. stir in vanilla. swirl cream over pie filling and up against meringue crust. Dust pie with nutmeg.
Chill 3 hours.