We must remember that the Roman Empire is the terrible fourth beast that Daniel saw, which the little stone (the Lamb who was slain) struck in its feet and toppled (Dan. 2:24-45 and Dan. 7). (The Sixth Seal: 313-325 AD, pg. 37).

Daniel’s interpretation and Nebuchadnezzar’s dream of a colossal statue is found in Dan 2.

Daniel’s vision of the four beasts is found in Dan 7:

7 After this I saw in the night visions, and behold a fourth beast, dreadful and terrible, and strong exceedingly; and it had great iron teeth; it devoured and broke in pieces, and stamped the residue with its feet; and it was diverse from all the beasts that were before it; and it had ten horns.
8 I considered the horns, and, behold, there came up among them another horn, a little one, before which three of the first horns were plucked up by the roots; and, behold, in this horn were eyes like the eyes of a man, and a mouth speaking great things.

Dan 7:7-8, Jewish Publication Society Bible, 1917.

“The world power in its totality appears as a colossal human form: Babylon the head of gold, Medo-Persia the breast and two arms of silver, Græco-Macedonia the belly and two thighs of brass, and Rome, with its Germano-Slavonic offshoots, the legs of iron and feet of iron and clay, the fourth still existing.”

Jamieson, Faussett, and Brown’s Commentary on Daniel 2:31.

“Whereas the three former kingdoms were designated respectively, as a lion, bear, and leopard, no particular beast is specified as the image of the fourth; for Rome is so terrible as to be not describable by any one, but combines in itself all that we can imagine inexpressibly fierce in all beasts. Hence thrice (Da 7:7, 19, 23) it is repeated, that the fourth was ‘diverse from all’ the others.”

Jamieson, Faussett, and Brown’s Commentary on Daniel 7:7.

The events of the first four seals, popularly known as the four horsemen of the apocalypse, are closely linked in meaning and time, for the same symbol, horses and horsemen, are used for all four (The First Four Seals: 96-300 AD, pg. 23).

“The first four symbols are connected by using the same symbol. In the total scheme of all the symbols, this style—making the first four in each group of seven to be connected—continues in the trumpets and bowls.”

Fred Miller, Revelation: A Panorama of the Gospel Age, Ch. 3, “The Opening of the Seven Seals.”