Can I start using Wegovy from a dose of 0.5 mg?

You can do it this way. If you have any digestive system disorders (diarrhea, vomiting), it is worth consulting a doctor. It may be necessary to go down to a dose of 0.25.

The manufacturer recommends the following dosage of the drug:
"Wegovy starts with a starting dose of 0.25 mg once a week. Your doctor will gradually increase your dose every 4 weeks until you reach the recommended dose of 2.4 mg once a week. Once the dose of 2.4 mg has been reached, further dose escalation is not recommended."

Here you can check the availability of this medicine in pharmacies and make a reservation:

https://www.gdziepolek.pl/produkty/115667/wegovy-iniekcja/apteki

Can you stop at the 4 pen without reaching that of 2.4?

Up to @Garghy:

Direct switching from Saxenda to Wegovy at a dose of 0.5 mg is not standard of care. It is usually recommended to start with the lowest dose of Wegovy (0.25 mg) and increase it gradually, regardless of previous Saxenda use. Dosage decisions should only be made by the treating physician after careful evaluation of the individual patient's case. It is crucial to exercise caution and monitor potential side effects closely when switching medications. Always follow your doctor's instructions and do not modify your treatment regimen on your own.