High-side switch 24V 6mA control by Arduino
Start by working back from the output. Collector current is only 4mA so suppose we allow base current to be Ic/Ib = 10 or 400uA. That will ensure it is well saturated. So R3 should be (24V - 0.7V)/0.4mA = 58K, so you can use 56K. 

R2 is just to take care of leakage in Q2. It is 50nA maximum at 25°C. If we assume operation to 95°C and doubling every 10°C then we have to allow for 6uA. If the base-emitter voltage of Q1 should be less than 300mV with 6uA of leakage, the R2 should be 50K or lower. We can use 47K for R2. Might as well make R3 47K too and keep the values the same. 

Collector current of Q2 is about 0.5mA. If we use the Ic/Ib = 10 again, and assume it should work with 4V input, then R1 &lt;= (4V-0.7V)/0.05 = 66K. We can use 47K again and keep all three the same.