this can be caused by: 1) change in season / weather patterns. 2) a change at the tower 3) possibly connecting to a more distant tower 4) Clear overlaying LTE 4g in some markets / beta testing it 5) other inference source / blocking source such as adjacent office moved metal cabinets around.
If its not causing speed to tank 4 lights is still acceptable signal level. I have had two or three lights during bad storms and it has been usable. You DO want to put the modem back exactly the way you had it when it got five lights assuming nothing else has changed. The signal may come back up to 5 lights on its own . If this gets worse and starts going 2 or 3 lights AND weather is not the cause then there is likely a tower issue.
The corner reflector is interesting. I have the modem "hidden in plain site" with a smaller cardboard box. I could line two of the boxes inside walls with foil but they would be quite close to the modem. If you look at the attached photos the signal comes in from the right perpendicular to the modem. The box is cut open on top and back to allow ample air circulation. It is amazing how well this little disguise works. My immediate co-workers knew I had the modem but when I asked them to guess where I keep it most of them did not guess it was in the box until I actually showed them. J