Re i4
Hi I have not owned an i4. I do have a i4 wheel spare for a project which has not yer been precieved yet!!
Afaik their us potential in the i4.
Fitting a twin chainring? Seems possible if you forgo the chainguard. And as you mention the i4 has more overdrive potention
You just need a front mech mounting bracket. Which will be either a dahon brommie or custom.