Praise be to Allaah.
And many saheeh reports indicate that what is prescribed for the Muslim when he comes to the mosque on Friday is to pray whatever Allaah wills he should pray before the imaam comes out. The Prophet (peace and blessings of Allaah be upon him) did not specify whether it should be two or four or more. All of that is good, but the minimum is two rak’ahs to “greet the mosque.”
With regard to after Jumu’ah, there is a regular Sunnah prayer, the minimum of which is two rak’ahs and the maximum is four, because the Prophet (peace and blessings of Allaah be upon him) said, “Whoever among you prays after Jumu’ah, let him pray four (rak’ahs)” (narrated by Muslim, al-Jumu’ah, 8881). And he (peace and blessings of Allaah be upon him) used to pray two rak’ahs after Jumu’ah in his house. May Allaah help us all to do that which pleases Him.