An international SMS is sent each time you activate iMessage/FaceTime(One SMS Only); International SMS Charge by your Carrier

If your iPhone is used with a carrier not found in the wireless carrier articles, an international SMS is sent each time you activate iMessage/FaceTime. (One SMS Only)

If there are 4 SMSs that indicates you have activated iMessage 4 times.

Each time you change the SIM or activate iMessage/iCloud/FaceTime you must have got these warnings (As Shown in the pic below). If you had tapped on OK then only then an SMS is sent .....

You will be charged by your Carrier

See the pic below

If you wish not to incur international charges, please do not activate iMessage for your phone number, as shown in the pic below.

This is a one-time charge for activation by your Carrier. Your iMessage would have been free for life. Now that you have de-activated, it will be charged again by your Carrier when you activate Message/FaceTime.

Read this Apple Article: If you can’t turn on or sign in to iMessage or FaceTime - Apple Support (IN)

An extract from the above article of it is given below...

[Check your device settings:

Make sure that you’re connected to a cellular data or Wi-Fi network. If you're using an iPhone, you need SMS messaging to activate your phone number with iMessage and FaceTime. Depending on your Carrier, you might be charged for this SMS.]