Thank you for posting your query in the Community. We need more information to correctly answer your question. Did you incur charges to text a 4 digit number? Or did you incur charges to the service offered? Most of the time sending the text itself would be a local text cost and you will incur charges to the service being offered.
@Community - Please join this conversation and share your expertise, thank you.
This question isn't due to a charge. It more inquiring because Google release a new chat like hangouts but you can use this new client to talk to people with sms but your message will go to a gateway/relay before it gets send to the recipient.
Thank you for the clarification. Are you referring to Allo chat? You may want to check with the support team of that service. I haven't tried that app yet. If it's similar to Hangouts then text message rates of the carrier will apply. If the service is like Whatsapp then it will use your data if you are not on WiFi etc. I recommend to get the clarification from the new chat support, thank you.
Even if it's a regular relay we cannot verify the charges associated with a "generic" app. To be safe we recommend using the service at your own discretion, and simply monitor your next invoice to see if you've incurred any applicable charges. I would also recommend you to read up on the FAQs for the app.
Unless you are texting a Canadian number there is no way to guarantee that you will not be billed.