mjohnson45
First thing I would try is just restarting the router. (helps to refresh network connections).
If still having issues, I would see if changing the band I was connecting to helps. If your Express is model 3960 then you can use either the 2.4ghz band (stronger signal, but may be more prone to interference), or the 5ghz band (a bit weaker in its range, but usually faster and less prone to interference).
You can determine your Express model number from Settings/System/About.
If any other model other than 3960, you are limited to using the 2.4ghz band and will instead just have to enter the router wifi settings and try changing the "Wireless Channel" to see if one performs better than the other. (ch. 1, 6, and 11 are recommended initially, but you may end up finding that another channel other than these works best in your environment).
Another test is to just move the device to one of your working Express 4k's (3940/3941) location and see if it performs better there. If so, then likely it is an interference issue that hopefully an adjustment to the wireless channel will fix.