Assalaamu alaykum. If I miss the Thuhr prayer with the congregation in the masjid, am I still able to pray four units of prayer before praying the Thuhr prayer by myself, or is it too late? For example, the prayer in congregation is at about 11:50, but if I miss the group prayer and pray at 12:30 by myself, can I still pray the additional voluntary units of prayer before praying my Thuhr prayer?

All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

If you miss the congregational prayer and there is enough time to perform the regular Sunnah prayer and the obligatory prayer as well before the time of the prayer expires (i.e. before the time of the next prayer starts), then you may perform the Sunnah prayer and then the obligatory prayer. An-Nawawi wrote about the regular Sunnah prayers that are performed before the obligatory prayers, "... it may be performed at any time before the time of the obligatory prayer expires; however, it is recommended to perform it before the obligatory prayer (if there is time for both)."

If there is not enough time to perform the Sunnah and the obligatory prayer, you should perform the obligatory prayer first and then make up for the Sunnah prayer. If you performed the Sunnah prayer first in this situation, it does not count as a valid regular Sunnah prayer. The Hanbali scholar Al-Buhooti said, "...and a voluntary prayer, even if it is a regular Sunnah prayer, is not valid if there is no time to perform both the obligatory and voluntary prayers. Such a voluntary prayer, if performed, is not valid because it is prohibited to perform a voluntary prayer in this situation, like the case with performing a prayer at the forbidden times for prayer because it is incumbent to perform the obligatory prayer instead..." [Kashshaaf Al-Qinaaʻ ʻan Matn Al-'Iqnaa']

The author of Mataalib Uli An-Nuha said, "If there is not enough time to perform the obligatory prayer in full, then it is incumbent to perform the obligatory prayer (first), and it is prohibited to perform the regular Sunnah prayer (first) in this case, because the time must be used for the obligatory prayer..."

Allah knows best.