4th tower's the proper siege tower, even in WB it was a staple. For BL, even on SP, not having it hurts because there's little to no advantage on using them other than providing partial cover and avoiding body blocking for attacking troops. Though defending troops AI is so incompetent that in the end makes not much of a difference, in fact most of the time it's faster and more reliable to just use default ladders, specially if the defending side has ballistas deployed...Once upon a time... a boy with high hopes...
I don't say for SP... it seems that Taleworlds has absolutely no intention to re/implement it but in MP siege environments for custom servers this kind of tower is VERY necessary. @Errayn Have you done any testing on your server? Is this tower still viable?
Sieges in BL don't make much sense realistically speaking. The best functionality for such a siege equipment's to provide cover for troops to climb walls much aligned with the principle of a "trojan horse", 2nd and 3rd tower fail at that, first does a better job because they can climb faster.