@LH865 

The issue is going to be that the Roku's are connected to the 2.4GHz band.  At best the 2.4GHz will achieve 150Mbps transmit rates, but that is in ideal circumstances.  In most cases you are looking at best rates between 80 Mbps and 100 Mbps.  With 4K streaming coming in at 4.3 to 7 GB per hour per device you can see how that could be significantly hampered by on the 2.4GHz  network.   Even though say Netflix's minumum recommendation for 4K is 25Mbps for just a single device, you also have to consider that that is in an indeal circumstance with no other devices drawing network resources or any interference from other 2.4GHz networks.   2.4GHz specturm is very congested, espically if you live in a large city or in a apartment complex.  

Ideally espically with 2 4K devices your best bet is putting them on the 5GHz band that is designed for higher data usage and has much less interference.  The downside is the Range is not as good.  But as long as the signal is there this is your best option without additional hardware.