HID bulbs can be bought in H7 base. Lots of kit makers sells them all kinds of qualities. They are all rebased from a D2S bulb.
Yes, you can do the math and make adapter for a H4HID bulb to match to H7.
http://faq.auto.light.tripod.com/special-topics.htm has the sizes of both H4 and H7, so u would have to make a detailed plan on how you would mount the H4 bulb.
I don't know if you have seen the list over Hella 90mm choices on
http://faq.auto.light.tripod.com/hella-90mm/ , but u might have noticed that the H9 version of hella 90mm is a better choice than H7 version, if you want to upgrade to HID bulb.
I'd recommend to sell the H4 HID kit u have now, buy H9 version of Hella 90mm, 2 Philips bulbs and hella ballasts. You will be happy, even if you have to create a bulb retainer (also described on above page) for the D2S bulb.