We don't usually consider most of the slide into rebellion from 1765-1775 as part of a 4T, but we are proposing to do just that with the 1852-1861 slide into civil war. The reason why is the 18th century 4T is long enough without having to make this addition while the 19th century 4T is not. I don't think trying to make the lengths work out is a valid reason.
The reason why S&H chose the bloodless Tea Party and not the Boston Massacre as their 4T start point was because it was only then that the British lost their patience and got tough, forcing the issue. Now when did the same thing happen for the Civil War. Certainly not in 1850, when the Compromise seems to calm the situation down. But then this was under a Whig administration. What happened after Democrats came to power in 1852? Well s
henanigans for one. The Kansas-Nebraska Act was a classic dodge on the issue, let "popular sovereignty" decide the issue. The casualties that resulted from Bleeding Kansas domestic violence were of a similar magnitude to those from domestic terrorism in the 1990's or the New Era 3T (
1,
2). I don't think violence by itself, even on a 911 scale, is enough to claim a 4T trigger.
Then there is the 1856 election. The party that represented the faction with the advantage in the correlation of forces (and who would eventually win the war) LOST the election. Instead they elected Buchanan as an "old hand" who would be able to keep things together. Clearly as late as 1856 there was no sense of an inevitability of war, the spiral (as Bob Butler would put it) had not yet reached its 1773 point. The came Dred Scott, and the situation became untenable, but violence did not break out, exactly when it would was still undecided. Americans still decided to rely on the ballot box. As the 1860 election progressed sentiment in parts of the South moved in favor of open rebellion if Lincoln should win. And when he did, war became inevitable. It seems clear to me that 1860 was the 1773 point in the Civil War spiral.