For 2 of the walls, Elijah will need 2 walls * 6 meters each = <<2*6=12>>12 meters of masking tape.
For the other 2 walls, he will need 2 walls * 4 meters each = <<2*4=8>>8 meters of masking tape.
He will therefore need a total of 12 + 8 = <<12+8=20>>20 meters of masking tape.
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