Avoth, chap 5, sec. 19.
There are four marks by which one disciple differs from another:—One learns and does not teach, one teaches and does not learn, one learns and teaches, and one neither learns nor teaches.
Avoth d'Rab. Nathan, chap. 29.
Four things, if kept in view and gravely pondered over, deter from sin:—That a man consider whence he cometh, whither he goeth, who the judge will be, and what the future will bring to pass.
Derech Eretz, chap. 3.
What is the meaning of that which is written (Ps. lxxxvii 2), "The Lord loveth the gates of Zion more than all the dwellings of Jacob?" The answer is, The Lord loveth the gates that are marked with the Halachah more than the synagogues and the schools; and this agrees with what Rabbi Cheeya bar Ami has said, in the name of Ulla, that since the destruction of the Temple nothing else has remained to God in His world but four ells of the Halachah.
Berachoth, fol. 8, col. 1.
Whoso walks even four ells with a proud unbending gait is as though he spurned with his haughty head the feet of the Shechinah; for it is written (Isa. vi. 3), "The whole earth is full of His glory."
Ibid., fol. 43, col. 2.