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

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?

#2 means #4

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

Out of the given options, #1 and #4 have the same meaning, as they both imply that an ambulance is used to transport someone to the hospital.

In #1, "using an ambulance" is a participial phrase describing how the action of taking someone to the hospital is achieved. This can be understood as "We usually take them to the hospital, and in doing so, we use an ambulance."

Similarly, in #4, the comma before "using" is used to separate the participial phrase "using an ambulance" from the main clause. So, you can interpret #4 as "We usually take them to the hospital (using an ambulance)."

As for #2 and #3, they have different interpretations:

In #2, "as/while we use an ambulance" implies the transportation to the hospital and the use of the ambulance happen simultaneously. So, it means "We usually take them to the hospital at the same time that we are using an ambulance."

In #3, "which use an ambulance" suggests that the hospital itself uses an ambulance. This interpretation is less likely because the subject of the sentence seems to be the individuals who are taken to the hospital, rather than the hospital itself.