Let's look at the numbers.

It is generally accepted that a plane with a 60-size 2-stroke would need a 90-size 4-stroke.

The OS 61FX has a displacement of .607 Cubic Inches
The OS 91 Surpass II has a displacement of .912 Cubic Inches

The 61 will run at approximately 11,500 RPM
The 91 will run at approximately 9,500 RPM

This means that for every rotation of the 61 it takes in .607ci of fuel/air mixture, or 6980.5ci per minute.
Meanwhile the 91 takes in .912ci of mixture every OTHER rotation, or 4332ci per minute.

6980.5 - 4332 = 2648.5

So the 91 is using less than 1/3 of what the 61 is using... AND this does not take into account the amount of fuel loss generated by a 2-stroke (For a 2-stroke to operate, the new mixture blows out the exhausted mixture and some of the new mixture is blown out along with it - which is one reason that 2-strokes slime your airplane so badly).

If you burn 3 gallons of fuel per month at $20/gallon, you will save $20 a month

Cost of a 61FX = $180
Cost of a 91S = $300

So in 6 months, the 4-stroke will have paid for itself, kept your airplane cleaner, made your airplane sound better, and in all probability, contributed to world peace.

So not even taking mixture loss into account, a 91 4-stroke burns about 1/3 less fuel that a 61 2-stroke, it swings a bigger prop, and it doesn't spit slime all over your plane.