A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown  at sacred-texts.com
gen 4:1BIRTH OF CAIN AND ABEL. (Gen. 4:1-26)
Eve said, I have gotten a man from the Lord--that is, "by the help of the Lord"--an expression of pious gratitude--and she called him Cain, that is, "a possession," as if valued above everything else; while the arrival of another son reminding Eve of the misery she had entailed on her offspring, led to the name Abel, that is, either weakness, vanity (Psa 39:5), or grief, lamentation. Cain and Abel were probably twins; and it is thought that, at this early period, children were born in pairs (Gen 5:4) [CALVIN].
gen 4:2Abel was a keeper of sheep--literally, "a feeder of a flock," which, in Oriental countries, always includes goats as well as sheep. Abel, though the younger, is mentioned first, probably on account of the pre-eminence of his religious character.
gen 4:3in process of time--Hebrew, "at the end of days," probably on the Sabbath.
brought . . . an offering unto the Lord--Both manifested, by the very act of offering, their faith in the being of God and in His claims to their reverence and worship; and had the kind of offering been left to themselves, what more natural than that the one should bring "of the fruits of the ground," and that the other should bring "of the firstlings of his flock and the fat thereof" [Gen 4:4].
gen 4:4the Lord had respect unto Abel, not unto Cain, &c.--The words, "had respect to," signify in Hebrew,--"to look at any thing with a keen earnest glance," which has been translated, "kindle into a fire," so that the divine approval of Abel's offering was shown in its being consumed by fire (see Gen 15:17; Jdg 13:20).
gen 4:7If thou doest well, shalt thou not be accepted?--A better rendering is, "Shalt thou not have the excellency"? which is the true sense of the words referring to the high privileges and authority belonging to the first-born in patriarchal times.
sin lieth at the door--sin, that is, a sin offering--a common meaning of the word in Scripture (as in Hos 4:8; Co2 5:21; Heb 9:28). The purport of the divine rebuke to Cain was this, "Why art thou angry, as if unjustly treated? If thou doest well (that is, wert innocent and sinless) a thank offering would have been accepted as a token of thy dependence as a creature. But as thou doest not well (that is, art a sinner), a sin offering is necessary, by bringing which thou wouldest have met with acceptance and retained the honors of thy birthright." This language implies that previous instructions had been given as to the mode of worship; Abel offered through faith (Heb 11:4).
unto thee shall be his desire--The high distinction conferred by priority of birth is described (Gen 27:29); and it was Cain's conviction, that this honor had been withdrawn from him, by the rejection of his sacrifice, and conferred on his younger brother--hence the secret flame of jealousy, which kindled into a settled hatred and fell revenge.
gen 4:8And Cain talked with Abel his brother--Under the guise of brotherly familiarity, he concealed his premeditated purpose till a convenient time and place occurred for the murder (Jo1 3:12; Jde 1:11).
gen 4:9I know not--a falsehood. One sin leads to another.
gen 4:10the voice of thy brother's blood crieth unto me--Cain, to lull suspicion, had probably been engaging in the solemnities of religion when he was challenged directly from the Shekinah itself.
gen 4:11now art thou cursed from the earth--a curse superadded to the general one denounced on the ground for Adam's sin.
gen 4:12a fugitive--condemned to perpetual exile; a degraded outcast; the miserable victim of an accusing conscience.
gen 4:13And Cain said . . . My punishment is greater than I can bear--What an overwhelming sense of misery; but no sign of penitence, nor cry for pardon.
gen 4:14every one that findeth me shall slay me--This shows that the population of the world was now considerably increased.
gen 4:15whosoever slayeth Cain--By a special act of divine forbearance, the life of Cain was to be spared in the then small state of the human race.
set a mark--not any visible mark or brand on his forehead, but some sign or token of assurance that his life would be preserved. This sign is thought by the best writers to have been a wild ferocity of aspect that rendered him an object of universal horror and avoidance.
gen 4:16presence of the Lord--the appointed place of worship at Eden. Leaving it, he not only severed himself from his relatives but forsook the ordinances of religion, probably casting off all fear of God from his eyes so that the last end of this man is worse than the first (Mat 12:45).
land of Nod--of flight or exile--thought by many to have been Arabia-PetrÃ&brvbr;a--which was cursed to sterility on his account.
gen 4:17builded a city--It has been in cities that the human race has ever made the greatest social progress; and several of Cain's descendants distinguished themselves by their inventive genius in the arts.
gen 4:19Lamech took unto him two wives--This is the first transgression of the law of marriage on record, and the practice of polygamy, like all other breaches of God's institutions, has been a fruitful source of corruption and misery.
gen 4:23Lamech said unto his wives--This speech is in a poetical form, probably the fragment of an old poem, transmitted to the time of Moses. It seems to indicate that Lamech had slain a man in self-defense, and its drift is to assure his wives, by the preservation of Cain, that an unintentional homicide, as he was, could be in no danger.
gen 4:26men began to call upon the name of the Lord--rather, by the name of the Lord. God's people, a name probably applied to them in contempt by the world.