The transitional event from the Third Age to the Fourth, judging from the tapestry of the World Hymn, seems to be the Great Dying or the invasion of the Orcs and Goblins. The transitional point between the Fourth and Fifth Age seems to the elven civil war.
It has been said that the fourth age was around 2000 years ago. Given that the Ninth Age is 962 years long so, this means the 5th, 6th, 7th and 8th Age combined were about 1000 years long. 
These are notable events taking place in the Fourth Age, listed (approximately) chronologically:
- Orcs and Goblins as well as Beast-men appear in great numbers.
- The Highborn Elves of Vetia continued to suffer from conflicts with the dwarves and barbarians (Orcs and Beasts ), causing many to emigrate. They are said to have been denied entry to Celeda Ablan, so instead they migrated to the colonies in Silexia.  The sources do not specify this taking place in the Fourth Age, but the World Hymn poem suggests so ("From elves disloyal no aid was found, ‘cross endless sea retreat and flee.")
- The greater Dwarven Empire is split in two again, western dwarves (today known as Dwarven Holds) and eastern dwarves (today known as Infernal Dwarves). 
- The human empire of Avras is founded.
- The dwarves ally with Avras.
Unless otherwise specified, text that green italic in the list above uses the World Hymn poem and tapestry as its source.
The World Hymn Edit
Main article: World HymnThe World Hymn is a dwarven carving of a poem that details all the nine ages of the world. It was later copied to an Equitan tapestry and supplemented with imagery. A transcript of the poem and the image for the fourth age are both shown below. 
An Age of Ruin, an Age of Iron
As split was world so too were we
Our only hope a foreign truce
With Avras proud we did ally
To war ‘gainst fate we did return
From elves disloyal no aid was found
‘Cross endless sea retreat and flee
Unfit they are of dwarven aid
Not worth the fire of dwarven rage
The path they tread is ruin bound
Of orcs of men of beasts no end
Now war and siege and misery
As one we kill so sprout two more
And flight nor fight avail a dwarf
Our dominance so too now rent
Endless foes did split us sunder
The orc and beast our gates did press
The hold’s rings split like maille from axe
No valour won could mend the loss
As foreign blades took our plunder
Our kith and kin no longer free
Our axe and shields unequal were
Our vengeance sworn forever be
- ↑ 1.0 1.1 1.2 The World Hymn: the 4th Age (T9A:FB Rulebook, v1.3.5, p128)
- ↑ Post by Calisson
- ↑ 3.0 3.1 Account by Rodomonte (The 9th Scroll, issue 9, p21)
- ↑ Informative text about the Dread Elves (The 9th Scroll, issue 6, p26)
- ↑ Informative text about the Infernal Dwarves (The 9th Scroll, issue 7, p38)