The flashiest comparison revolves around the Glyph interface - it's a simple Glyph bar on the non-Pro and a full-on circular dot matrix display on the Pro. Getting technical for a bit, the Pro has 137 LEDs, while the regular has just 63. As you can imagine, you can do all sorts of clever stuff with the Glyph Matrix on the Nothing Phone (4a) Pro - animating your timer, different callers, you can even use it as a selfie display for the main camera.
It features an aluminum unibody while retaining Nothing's retro-clear hardware design touches, with a clear, redesigned camera unit. Yes, the aggressively protruding circular camera unit of the Phone 3a Pro is gone, replaced with an oblong housing that houses the triple-camera array and a tweaked Glyph Matrix, similar to what debuted on last year's Nothing Phone 3.
The Nothing Phone (4a) Pro drops (3a)'s plastic frame and brings a metal unibody design. It's also slimmer at 7.95mm (down from 8.4mm) but weighs the same (despite using metal instead of plastic). The phone is rated IP65 - Nothing couldn't quite hit the requirements for IP67, so it claims that the phone can survive submersion down to 25cm for 20 minutes.