A: Thank you for your inquiry. When using 2x material, you will be using a 2-1/2" screw.
A: If both boards are 2x4’s (1 1/2” thickness), you should use 2 1/2” screws. Here’s a good link for a reference: https://industrydiy.com/woodworking/kregtools/kreg-jig-settings-chart/
A: The K5 is larger and meant for bigger pieces of wood and it also has a built in clamp and dust cover. The K4 is moreso for DIYers and smaller. Both are great.
A: It will. I have used this many times with 2x4s and they are solid joins.
A: Given the depth settings and clamp depth it will certainly create pocket holes in 2x4 lumber.
A: Thanks for the questions. The Kreg K4 will work with your standard 2x4 that is 1 1/2" thick.
A: Yep! I’ve used it on 2x4s plenty of times.