It’s recommended only 3 units can be connected to W21, wherein one can be used as a Main Router and the other two as Satellites.

Is the blinking red twice and green once happening on the 4th unit or the Main Router or on either of the Satellites? 

W121 is termed as a “Dual Pack” and W21 a “Single Router”.

Yes. My mistake. I realize now. I bought two two packs. Is there a way to add the fourth unit? I really need the coverage. I have a garage that is seventy feet from my deck. I need better coverage out there. I have a third satellite in the carport attached to the garage and it helps. 

Thanks,

Dave

With the help of the third satellite try connecting to the garage and carport to get the coverage.

As mentioned earlier It is recommended that only 3 routers where 1 can be Main and the other 2 as satellites can be connected to give good coverage. 

The only option for the  4th unit is it can be used as a main Router by connecting to a modem only.

Are you saying that I could run two separate routers with satellites? Will it work efficiently?

As mentioned earlier It is recommended that only 3 routers where 1 can be Main and the other 2 as satellites can be connected to give good coverage. 

Hence, It won't be possible to connect two separate routers with satellites as it requires 2 cable modems and  2 co-axial connections.

You can only have 3 max routers in a mesh network.  Can’t add the forth. 

It is too bad. System works great! It could work better if I could add more satellites. But I like this system. It works better than the extender systems I have been using.