I bought MyBook single drives which contain the WD80EZZX drives, not Red drives. The MyBook Duo's have Red drives (and are easier to open).
If you want 4TB Red drives, the 8TB Duo is currently in stock... $349.99 for two drives, minus 15% = $297.49 = $148.75 per 4TB Red drive before tax (which is more than the OP's price listed above, so in this case shucking probably doesn't make sense).
For the record, my price was $232.68 after 9.5% sales tax ($212.50 before). My price on 8TB Reds going the Duo route would have been around $256/drive after tax ($233.75 before). I might have bought 16TB Duos if I could have but they were and still are out of stock.