Hi Haruna, MTN do automate their postpaid bill payment process, you are not supposed to be suspended for 4 days after paying your monthly bills. You need to contact MTN support with the affected phone number and your channel of payment details for more investigation and resolution.
You can contact MTN support via any of these channels:
1. MyMTN App
2. Facebook: @MTNLoaded
3. Instagram: @MTNNG
4. Live Chat: http://bit.ly/2OC5bHz
5. Email: customercareng@mtn.com
6 .WhatsApp: 09033000001