You can either use the session-wide link, persistent links, or single-use links.

Session-wide link

If it is impractical to distribute distinct URLs to each participant, you can give the following start URL to all 4 participants.

https://floating-sierra-61355.herokuapp.com/join/i7xdj66gai/

Note: unlike the other link modes, this does not prevent the same person from playing twice.

Persistent links

If you want to give your participants permanent links that don't change, you should create your session in a room.

Single-use links

Below are single-use links, which you can distribute to your participants. Each link has a unique code for the participant.

P1	https://floating-sierra-61355.herokuapp.com/InitializeParticipant/ciil7yhg
P2	https://floating-sierra-61355.herokuapp.com/InitializeParticipant/usf7ai3k
P3	https://floating-sierra-61355.herokuapp.com/InitializeParticipant/rdfx95gc
P4	https://floating-sierra-61355.herokuapp.com/InitializeParticipant/98b5c76i