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You can make a substitution into p_4 and plot the result, e.g.:

plot(p_4.subs(d_e == 0, d_k == 1/2, d_m == 1, e_m == 1, e_s == 1), (x,-5,5))

The substitution into p_4 returns a new expression, and does not alter p_4 itself.