You can't get 1X out of a disc that says 4X is the slowest it will work. The drive cannot be forced to work at a speed not acceptable to the disc manufacturer.
Just on this, ImgBurn rates my TDK DVD-R disks at: "Supported Write Speeds: 6x, 8x, 12x, 16x" whereas the container they came in states "1-16x speed v2.1 rev.6.0".
Consequently, like the OP, when I selected 1x write speed the actual write speed was logged as "Average Write Rate: 8,086 KB/s (5.8x) - Maximum Write Rate: 8,428 KB/s (6.1x)"
[FYI: I want to burn the Win7 RC at 1x speed for reliability reasons]