4:6 and before the throne there was, as it were, a sea of glass like crystal; and in the center and around the throne, four living creatures full of eyes in front and behind.

and before the throne there was, as it were, a sea of glass like crystal;

In the court of the Old Testament temple, a "sea" was made for the priests to bathe in (1 Kings 7:23-26). The size was about 17 1/2 feet in diameter, 8 3/4 feet deep, and empty weight estimated between 25-30 tons. With water, its weight was around 100 tons and its capacity was between 16,000 to 20,000 gallons.

The "sea" was located between the altar and the tent of meeting. The priests were required to cleanse themselves prior to entering the tent of meeting. The consequence if not bathed was death. See Exodus 30:17-21.

Symbolically, today we, the true believers, are the royal priesthood: 1 Peter 2:9-10, Revelation 1:6, and Revelation 5:8-10. Like the priests in the Old Testament, we, too, must be clean, 2 Timothy 2:21. Furthermore, since Christ's blood "cleanses us from all sin" (1 John 1:5-10), He is our sea. This is confirmed by 1 John 5:6-12, Ephesians 5:25-27, and Hebrews 10:19-22; but this is another whole study in itself.

and in the CENTER and AROUND the THRONE four LIVING CREATURES full of eyes in front and behind.

These four LIVING CREATURES are probably the highest order of angels. They stand by the THRONE and execute the Lord's commands concerning the Tribulation on earth. They are present when the seals are broken, and they can speak, Revelation 6:1-8.

From Revelation 7:1, four is associated with the "four corners of the earth". They have eyes all around (verse 8) meaning full wisdom -- they know the entire history of the earth. They see/have seen the evil, and understand why they "guard" the THRONE (the seat of complete and perfect judgment).