Daily by the end of the Fitr prayer (in the third rakath) in the Sujooth position I used to make my dua for nearly 4 minutes. Is it OK? (I am the only man in the mosque following this way)

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

If what you mean by “in the third Rak‘ah in the Sujood position” is the final Sajdah, then prolonging the Du‘aa’ in that specific Sajdah, excluding the others, is not part of the guidance of the Prophet, sallallaahu ‘alayhi wa sallam. Even if making abundant Du‘aa’ is essentially legislated, such specification is not. The prayer of the Prophet, sallallaahu ‘alayhi wa sallam, was almost balanced. Al-Baraa’ said: "The Sujood and Rukoo‘ of the Prophet, sallallaahu ‘alayhi wa sallam, as well as his sitting between the two Sajdahs, were almost equal in length." [Al-Bukhari and Muslim]

You should leave this act, as the best guidance is the guidance of Muhammad, sallallaahu ‘alayhi wa sallam. Shaikh Ibn ‘Uthaymeen said: "Prolonging the final Sajdah is not part of the Sunnah, since the Sunnah is to have all parts of the prayer practically equal in length, i.e., Rukoo‘, standing after it, Sujood, and sitting between the two Sajdahs." This is as Al-Baraa’ ibn ‘Aazib, said: “I observed the prayer with the Prophet, sallallaahu ‘alayhi wa sallam, and I found his Qiyaam (standing), his Rukoo‘, his Sujood, and his sitting at the end to be almost equal in length.” This is best, but know that there is another place for Du‘aa’ other than Sujood, and that is the Tashahhud. The Prophet, sallallaahu ‘alayhi wa sallam, taught ‘Abdullaah ibn Mas‘ood the Tashahhud and said: “Then let him choose whatever Du‘aa’ he likes.” He thus made the Du‘aa’ permissible, whether short or long, after the final Tashahhud, prior to saying Tasleem (to end the prayer)." [End of quote]

Finally, it is best for you to distribute the Du‘aa’ throughout the different Sajdahs of the prayer, until each is similar in length to the others, and that you make Du‘aa’ after Tashahhud and before the Salaam.

Allaah Knows best.