The problem with evaluating speed in MGIV or any MC paper is multifold.
1. Which emulsion do you check for speed point? There are at least 2 in MGIV.
2. Since there are two contrasts, which emulsion do you develop to the ISO standard?
3. Since it a paper, how do you apply film rules.
4. If you develop to a gamma of 0.62, this is severe underdevelopment of a paper material therefore leading to low threshold speed estimation.
So, the practical problem as opposed to theoretical problem is "what do you do to use a paper as an in-camera negative material".
I chose to meter it as if it were a negative material, and therefore got an effective ISO which is in reality an EI of 25 for MGIV. So, if you take MGIV and put it in your camera you can expose it at EI 25 (ISO 25). It works. The pictures are not perfect, but they indicate that the ISO (EI) is close to 25. Endura paper, BTW, is about 25 also when filtered to daylight. I knew this for almost 30 years, having used it (type "C" paper then) many years ago, but the contrast is again high.
There is the balance between theoretical and practical knowledge. Theoretically, developed to a gamma of 0.62, MGIV is underdeveloped and has low speed, but deveoped to a gamma of 2.5 has a speed of about 25 and yields decent pictures in-camera when the negative is scanned and inverted. You can do this in the darkroom or using a digital scanner editor program. You may also wish to lower contrast by using a low contrast grade paper or by digital means. I have done both.
I have examples of most of this on-hand to post if desirable to members. I ask Ryuji to post some examples to further extend our knowledge of silver-gelatin. We have seen many of his comments, but practically none of his work. Seeing some of it would be most helpful to us all, I'm sure.