Short answer: your 4S data package starts on first use, when the eSIM connects and starts using mobile data.
Important difference
Installing the eSIM adds it to your phone. Activation starts the data package on first use.
Recommended timing
Install within 1 month before departure, then use the eSIM at your destination when you are ready for the plan to start.
Need help?
Contact 4S support through support@4s.net, a Zendesk support ticket, or Zendesk live chat. Do not reply to hi@4s.net; it is an automatic order and installation email sender.
When contacting support, include your order email, ICCID if available, destination, phone model, and screenshots of the error or SIM/eSIM settings page.