I had already uninstalled version 3 on both PCs and then installed it

Quoting ,

quoting post

I had a subscription to version 3 now I purchased a subscription to version 4 when I go to install it the primary computer cannot connect to the secondary computer ... I followed the various procedures that are indicated on your site but I always have the same problem of no connection ... I tried via cmd to do a ping and it's ok ... I also tried to format the PC but it cannot connect .... clicking on the test button gives me an error kindly help me .. thanks

This is the best resource available for troubleshooting this issue:

https://support.stardock.com/space/SHC/1287946260/Multiplicity+Connection+issues+101

Sean Drohan
Stardock Product Lifecycle Manager

Make sure you have set the same encryption code on both ends

Also make sure both ends are running v4 as v4 will not connect to a v3 client.  If you uninstalled v3 first did you also reboot before installing v4?

The firewall may need adjusting if you use a different firewall too.

I suspect the error you mean is this

If not, what error?

If you tried ALL of the options in the link I provided, including the port fix batch file, the next step is to eliminate any 3rd party app that could be causing the connection failure.  You can do this by doing a 'clean boot' on BOTH PCs

https://support.stardock.com/space/SHC/1346601039/Multiplicity+Clean+Boot

Once under the clean boot conditions (which MUST include being under a new Windows Administrator account - detailed in the documentation), please try again. 

Sean Drohan
Stardock Product Lifecycle Manager

If either connection is Wireless, try connect both via ethernet and ensuring they are both on the same subnet.

Sean Drohan
Stardock Product Lifecycle Manager

just for the sake of communication .... I uninstalled the v4 version and reinstalled the v3 on both PCs and everything is working

at this point I ask you why? I hope for a solution from you

thanks