1. We usually take them to the hospital using an ambulance.

Question

2. We usually take them to the hospital as/while we use an ambulance.

3. We usually take them to the hospital which use an ambulance.

4. We usually take them to the hospital, using an ambulance.
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Does #1 mean #2 or #3 in meaning?
What about #4. In #4, a comma is put before 'using.' Does #4 mean #2?

Answer 1

#2 means #4

Answer 2

Yes, #4 means #2, and yes, the comma is needed.