Possible Meaning Behind the 40 Years in 2 Samuel 15:7

Another accusation against the Bible is the use of the term forty years to describe the time it took Absalom to finally throw his dastardly rebellion against his father David. Some of the modern versions use four years instead of forty years to explain the rebellion of Absalom. It couldn't have happened literally that Absalom waited for literal forty years - otherwise David would have already been too dead to be rebelled against or to even launch an attack. So what would be a better suggestion for the forty years in 2 Samuel 15:7?

Here's a series of suggestions that could make sense about the "forty years" that took place before Absalom decided to rebel against his father:

• Absalom could have actually been 40 years old at that time. Considering that David was already 49 when he saw Bathsheba and 50 when he had Solomon. David was 30 when he began to reign and 70 when he died. If Solomon were growing up by that time then David would have been an old man by then. 
• John Calvin suggests that the forty years were actually the time when Israel asked for a king to the very day Absalom decided to rebel.
• 40 years is also an expression of David is now an elderly man.
• Also, some suggest that it was 40 years after David was anointed as king over Israel - which makes it all the more insidious for the ungodly Absalom to go against his godly father David!

The Baptist expositor John Gill also mentioned this one which could also make sense:

And it came to pass after forty years,.... Or four years; so long it was from the reconciliation of Absalom to David, as Josephus says; and so read Theodoret on the place, the Syriac and Arabic versions: but some say it was either forty years from the time Israel first had a king; and which might be an era of reckoning with the Jews, as the era of Seleucidae was with the Greeks, on the like account; or from the time Saul slew the priests at Nob, as Jerom; or from the time of David's being anointed by Samuel; or this was the year of Absalom's age, or of David's reign: but these, and other attempts made to account for this passage, are not entirely satisfactory; and therefore one may be tempted to conclude there must be a mistake in the copy, of "arbaim" for "arba", forty for four; which makes it quite easy, and confirms the first sense: 

that Absalom said unto the king, I pray thee, let me go and pay my vow, 

which I have vowed unto the Lord, in Hebron; not what he vowed in Hebron; for according to his own account he had vowed it in Geshur, as in 2 Samuel 15:8; but his request is, that he might pay it in Hebron; which place he fixed upon, being his native place, and where David was anointed king; and which, being about twenty miles from Jerusalem, was at a proper distance to lay the scene of his conspiracy in, and bring it to perfection. 

It can already be seen that the forty years isn't literal. Was it a scribal error? Some suggest that it was but isn't God's Word inerrant? The idea of four years may have been because it's the literal figure since Absalom rebelled. All the while the forty years represents a figurative number.