6:7 And when he had opened the fourth seal, I heard the voice of the fourth beast say, Come and see.

...I heard the voice of the fourth beast... This refers back to the fourth of the four beasts of the 1st 1/2 of the first set--the flying eagle. Could this eagle indicate, as per the other three beasts, that this seal will be especially devastating to the fowls of the air?

...say, Come and see...again, alluding to the Rapture that signaled the start of Daniel's 70th Week.

6:8 And I looked, and behold a pale horse: and his name that sat on him was Death, and Hell followed with him. And power was given unto them over the fourth part of the earth, to kill with sword, and with hunger, and with death, and with the beasts of the earth.

...a pale horse...the rider of this horse, Death, will be given power to kill with war, with famine, with disease and with the wild animals of the earth. (This will not be the first period of This half of Daniel's 70th Week that will be known for it's loss of life. The rider of the red horse in the last set was given a "great sword" and the power to take peace from the earth and cause men to kill one another.) Hell will follow the rider of the pale horse, reaping a windfall (one fourth of those alive at the time). (For foreshadowing of the four horsemen of Revelation, go to Zech. 6:1-8.)

6:9 And when he had opened the fifth seal, I saw under the altar the souls of them that were slain for the word of God, and for the testimony which they held:
6:10 And they cried with a loud voice, saying, How long, O Lord, holy and true, dost thou not judge and avenge our blood on them that dwell on the earth?
6:11 And white robes were given unto every one of them; and it was said unto them, that they should rest yet for a little season, until their fellowservants also and their brethren, that should be killed as they were, should be fulfilled.

--These cannot be those who were slain for their testimony during the Tribulation Period or during the 1st 1/2 of Daniel's 70th Week at the end of our age, for we are told that those will reign with Christ during the Millennium (Rev. 20:4).

Rev 20:4
And I saw thrones, and they sat upon them, and judgment was given unto them: and [I saw] the souls of them that were beheaded for the witness of Jesus, and for the word of God, and which had not worshipped the beast, neither his image, neither had received [his] mark upon their foreheads, or in their hands; and they lived and reigned with Christ a thousand years.

Also, the fact that they died violently establishes that they were not slain in the Kingdom Age during the time of Christ's rule, which is also known as the 1,000 years of peace--when no acts of violence will be permitted (Isaiah 9:7; 11:10 cf. Rev. 2:26-27). (Remember, the angels will be visibly ruling with Christ as will His glorified Church. Men and women may still be able to think evil, but for the 1,000 years, they will not be allowed to do evil because they will be living under an absolute theocracy.)