In generation 4 the Oak's Letter event was it ever possible to get it shiny that way?
Yes it was but you have to soft reset for one.
PS Flower Paradise Event I think to D/P/Pt
Yes it is possible to get any Pokemon as shiny even event Pokemon that require event items.
Because ones you get from event items aren't all the same however the ones without items that are just plain given to you are all the same right down to IVs.