We have here some of the genealogies of the tribe of Simeon (though it was not a tribe of great note), especially the princes of that tribe, Ch1 4:38. Of this tribe it is said that they increased greatly, but not like the children of Judah, Ch1 4:27. Those whom God increases ought to be thankful, though they see others that are more increased. Here observe, 1. The cities allotted them (Ch1 4:28), of which see Jos 19:1, etc. When it is said that they were theirs unto the reign of David (Ch1 4:31) intimation is given that when the ten tribes revolted from the house of David many of the Simeonites quitted these cities, because they lay within Judah, and seated themselves elsewhere. 2. The ground they got elsewhere. When those of this tribe that revolted from the house of David were carried captive with the rest into Assyria those that adhered to Judah were remarkably owned of God and prospered in their endeavours to enlarge their coasts. It was in the days of Hezekiah that a generation of Simeonites, whose tribe had long crouched and truckled, was animated to make these bold efforts. (1.) Some of them attacked a place in Arabia, as it should seem, called the entrance of Gedor, inhabited by the posterity of accursed Ham (Ch1 4:40), made themselves masters of it, and dwelt there. This adds to the glory of Hezekiah's pious reign, that, as his kingdom in general prospered, so did particular families. It is said that they found fat pastures, and yet the land was quiet; even when the kings of Assyria were giving disturbance to all their neighbours this land escaped their alarms. The inhabitants being shepherds, who molested none, were not themselves molested, till the Simeonites came and drove them out and succeeded them, not only in the plenty, but in the peace, of their land. Those who dwell (as we do) in a fruitful country, and whose land is wide, and quiet, and peaceable, have reason to own themselves indebted to that God who appoints the bounds of our habitation. (2.) Others of them, to the number of 500, under the command of four brethren here named, made a descent upon Mount Seir, and smote the remainder of the devoted Amalekites, and took possession of their country, Ch1 4:42, Ch1 4:43. Now the curses on Ham and Amalek had a further accomplishment, when they seemed dormant, if not dead; as had also the curse on Simeon, that he should be divided and scattered (Gen 49:7): yet to him it was turned into a blessing, for the families of Simeon, which thus transplanted themselves into those distant countries, are said to dwell there unto this day (Ch1 4:43), by which it should seem they escaped the calamities of the captivity. Providence sometimes sends those out of trouble that are designed for preservation.

These were their cities unto the reign of David - It appears that David took some of the cities of the Simeonites, and added them to Judah; Ziklag for instance, Sa1 27:6.
As the tribe of Simeon had withdrawn their allegiance from the house of David, the kings of Judah extended their domination as far as possible into the territories of that tribe, so that they were obliged to seek pasture for their flocks at Gedor, and in the mountains of Seir, as we find Ch1 4:39-42.

And Meshobab, and Jamlech,.... These, with those that follow to the end of Ch1 4:37 were famous men in the tribe of Simeon, of rank and dignity, and eminent for courage and valour, as the latter part of the chapter testifies, though they are nowhere else taken notice of. Jamlech, as Fabritius (k) observes, is not very different from Jamblichus, the name of a famous Platonic philosopher.

(k) Bibliothec. Gr. l. 4. c. 28. p. 294.

These were their cities unto the reign of David--In consequence of the sloth or cowardice of the Simeonites, some of the cities within their allotted territory were only nominally theirs. They were never taken from the Philistines until David's time, when, the Simeonites having forfeited all claim to them, he assigned them to his own tribe of Judah (Sa1 27:6).