Switching from IMac to 13"MBP, will I be happy with 4k performance?

Currently I have the 21.5" 3.1ghz quad core i7 16GB RAM Fusion Drive.

The NVIDIA GeForce GT 650M 512mb graphics card is holding me back: all 4k video files play jumpy and staggered. It can't handle playing them back.

I need to upgrade to edit and play 4K video files.

Just got back from the Apple store and there was some confusion there. I was not confident about what they told me.

How much of a speed reduction am I going to get going down to a 13"MBP with 2.7ghz i5 dual core vs. the 3.1ghz quad core I have now on Premiere Pro and other professionals apps? Noticeable?

I brought sample 4k files on a small USB 3.0 drive and they played silky smooth on the 13" MBP. But they didn't have premiere pro installed to try test rendering.

1. I want smooth 4K video playback of my files

2. I want portability of the 13" vs. the 15" MBP

3. I don't want a slow machine for exporting video files in Premiere Pro CC

4. I plan to get an external 4K display like the LG or Samsung when I am home editing