Albert Barnes Commentary 1 Samuel 4:18

"And it came to pass, when he made mention of the ark of God, that [Eli] feel from off his seat backward by the side of the gate; and his neck brake, and he died: for he was an old man, and heavy. And he had judged Israel forty years." — 1 Samuel 4:18 (ASV)

A comparison with 2 Samuel 18:4 explains the exact meaning of the “side of the gate” and Eli’s position. His seat, or throne, was without a back and stood with its side against the doorpost of the gate. This left the passageway clear but was placed so that everyone going through the gate had to pass in front of him.

Forty years – This chronological note connects this book with the book of Judges (compare to Judges 3:11 and similar passages). It is an interesting but very difficult question to determine how close Eli’s forty years as judge bring him to the time of the death of Phinehas, the son of Eleazar the High Priest. It is probable that at least one high priesthood intervened.