Your point is valid. In theory you could estimate OS version usage from browser statistics as well but I haven't seen useful data generated this way.
However the Adult Literacy XPRIZE field tests won't even start until the second late of 2017.l (currently scheduled for October 2017). An the issue of supporting multiple devices will become even more important in the cities deployment phase, due to start in December 2018.
By December 2018 I expect Android 4.0 to be extremely rarely used. In fact, given the timeline, I don't think that it would be particularly harmful to require a minimum Android version of 5.1 (Lollipop).
Given issues around browser compatibility, I think that asking for a minimum Android version of 4.1 (Jelly Bean) is a suggestion that will have minimal negative impact on the expected population of users while facilitating developers ability to take advantage of the latest (Chrome) browser features and security patches.