The source gives a separate rule for a four-rak'ah Sunnah:
• if the Khutbah begins during the pre-Jumu'ah Sunnah, or Iqamah begins during the pre-Zuhr Sunnah;
• complete 2 rak'ahs and give Salaam;
• join the Fard;
• then make up the remaining Sunnah after the Fard according to its proper rule.
The preferred course is not to continue all 4 and risk missing the congregational Fard.
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When Congregation Begins during an Individual Fard or Sunnah Prayer
The source gives a separate rule for a four-rak'ah Sunnah:
• if the Khutbah begins during the pre-Jumu'ah Sunnah, or Iqamah begins during the pre-Zuhr Sunnah;
• complete 2 rak'ahs and give Salaam;
• join the Fard;
• then make up the remaining Sunnah after the Fard according to its proper rule.
The preferred course is not to continue all 4 and risk missing the congregational Fard.