Okay so Surface Laptop is running Windows 10 S that only allows apps from MS Store though it's upgradable to Windows 10 Pro - Surface Pro 4 comes with Windows 10 pre-installed.
Surface Laptop supports touch and pen input - Surface Pro 4 supports touch and pen input though it has a kickstand that enhances the experience when using the pen on the screen as it prevents it from wobbling.
Surface Pro 4 has a detachable keyboard - Surface Laptop doesn't have a detachable keyboard.
Surface Pro 4 has an older Skylake CPU - Surface Laptop has a newer Kabylake CPU.
User-wise I think the Laptop is for the all-round user doing a bit of everything. The surface pro 4 is for a smaller niche of people such as creatives (artists, architects etc) and frequent travelers.
Well specs wise it's pretty much the same except that it has lower screen resolution, lower camera capture resolution and a newer processor than Surface Pro 4.
I would disagree, because it can be very useful for students too for taking notes, or professionals that are a lot on the move as you already mentioned. Sure it can't replace your desktop gaming rig, but it's a powerful device and can replace your laptop (if you are not a gamer or do a lot of photo and video editing). Specs wise it's on par with Surface Laptop and I'd say I would get that before Surface Laptop (even though I'd be happy to have the laptop too, lol) because of the kickstand which makes it a lot easier to use the Surface Pen.