What is the emitter resistor of Q2 supposed to be?
I couldn't find either of the transistors in my LTSpice library, so I substituted ones that have similar specs, especially Hfe.
The circuit is very sensitive to the actual Hfe of Q1 (therefore, IMHO not a very good circuit).
First figure shows the current limiting as a function of the load resistance, R4 with a WAG for R3. Red trace is the voltage output. Green trace is the current through R4. Note that for load resistance < 18.6Ω, the output current is ≈ constant at 20mA. For load resistance > 24Ω, the output current is inversely proportional to R4, so no current limiting there. Between those values, the fold-back is happening.
Second figure shows the effect of changing R3 from 2K to 3K to 4K (order: left to right). Note that R3 seems to effect the onset of current limiting, but doesn't effect the folded-back value of 20mA very much.
Third fig. shows the folded-back current as a function of Load resistance R4 while R2 is varied from 50K to 80K in steps of 10K (order: Green, red, lt. blue and dk. blue, respectively). The f.b. current is a function of how much base current flows out of Q1.
This should give you something to work with.