As of today, our standard 3" Benchrest Alignment Block comes with bolt-on rails that allow you to convert it to a 4" block. Once installed, these rails give the block the same footprint as Alex's 4" LRB, or a 3" stock that you converted to 4" via our bolt on rails. We have not updated the product listing yet, but I personally changed over our inventory this morning. If you want one, they are available here: https://fieldandcaveoutfitters.com/...tool-for-benchrest-set-up?variant=23656329857 Select the Flat Bottom Option, NOT the Catamaran. The Catamaran is for 3" stocks with rails machined into the block itself. This is my personal one, dinged up from field use. You should see the concept, though. Thanks for your continued support.