I seem to remember Stephen Seiler had a rough suggestion for how you can achieve reasonable fitness with 4 days a week by sticking to a semi-polarized approach. But, unfortunately, can’t find what the recommendation was (read it originally in his “MAPP” pages–which you can still find using the “Wayback machine”. Or in email correspondence he’d engage in before he became a star…)
I’m mostly interested in what the principles of designing such a program would be–say you built it around one “over-distance” workout and one moderate intensity interval session. Do you these both count as “hard” and then your other two days are very easy recovery efforts of moderate to low duration?