I have no idea about the 4k version, but I have two of the Roku 3 boxes. They are quite snappy, and the software on them (specifically the UIs for the streaming services) keeps getting better all the time.
The RF remote control is great because I can just hang the Roku on the back of my mounted flat screen TV.
The Roku 3 now has a slot for a microSD card:
https://support.roku.com/hc/en-us/articles/208756158-What-is-a-microSD-card-and-how-do-I-install-one-in-my-Roku-player-
The only other thing I recommend is ordering a Powerline kit to use instead of WiFi if you cannot run an ethernet cable. The speed difference is significant.