Q: rMBP 13" + 4k monitor? or just rMBP 15"

Hello,

I need to buy machine powerful enough to handle 2K video editing in Final Cut with colour grading and some minor effects in Motion, Photoshop with 40 MPX photographs having around 15-20 layers, and Lightroom scrolling thru 40 MPX RAW files fluently. I currently have base line 2014 rMBP 15" that handles those tasks well (although sometimes I miss dedicated GPU). This rMBP 15" just failed and I'm getting refund for it.

I could get 2015 base line rMBP 15" -- or -- in that price buy base line rMBP 13" + Dell 4K monitor. 13" would be much more portable and I need to care it every day for minor tasks at collage. Also, having large 4K screen at office to work would be fantastic. (and I hate 15" rMBP doing its spacer-rocket-lift-off sound to cool itself)

My only concern is whether 2015 base line 13" rMBP will handle all those tasks? I am fine with much longer final renders, but cannot wait for time line to render. Opening and saving times in PS could be longer but editing must be fluent with small brushes being used. And there is this 4K screen -- 13" rMBP should be able to handle it but could it be workable with all those apps?

Huge thanks for all the answers.