While this is quite late, hopefully this will help anyone else with a similar issue. The image_id needed for v4 of the API can be obtained using the image_key from v3 by replacing the pKey of an app URL. For example, given the image_key jts2GFO_3uyaEm5voOO5ug we can get the image_id by going to Mapillary
When going to that URL, you will be redirected to the same URL but pKey will be 195789302375444 which is the image_id. In a Python script, something like urllib can be used to open the URL. The returned URL can then be parsed to get the image id. Below is the Python code that implements this:
import urllib
image_key = ‘jts2GFO_3uyaEm5voOO5ug’
mapillary_URL = urllib.request.urlopen(‘Mapillary’ + image_key)
mapillary_url_as_string = str(mapillary_site.url)
image_id = mapillary_url_as_string.split(“&pKey=”)[1]
print(image_id)
There is probably a much better way to do this but this was the path I found.