Hi,
well, it's not true that improbable or impossible situations require the subjunctive... but I really don't know if the subjunctive is natural with "suppose" and "imagine". I'm afraid it's only ok with "suppose", so #4 is the only one that is unnatural. But I also think present tenses are much more common, so #1 and #3 are the most natural ones... Does this make any sense? Thanks