Step 1 : Selecting RB and C

To design above requirement, we need to use astable multivibrator where $T_{ON}$ = 4 sec and $T_{OFF}$= 2 sec.

$T_{OFF}$ formula is given by,

$$T_{OFF} = 0.69R_BC$$ Assume C = 100µf $$2 = 0.69R_B × 100µf$$ $$\boxed{R_B = 28.98KΩ}$$

Step 2 : Selecting $R_A$

$T_{ON}$ formula is given by, $$T_{ON} = 0.69 (R_A + R_B) × C $$ $$4 = 0.69 (R_A + 28.98KΩ) × 100µf $$ $$(R_A + 28.98KΩ) = \frac{4}{0.69 × 100µf}$$ $$R_A = 57.97KΩ - 28.98KΩ$$ $$\boxed{R_A = 28.99KΩ}$$

Step 3 : Designed Circuit Diagram