Which equation is a redox process?

#1.# #NaBr(aq) + HCl(aq) rightleftharpoons NaCl(aq) + HBr(aq)#

#2.# #HBr(aq) + AgNO_3(aq) rarr AgBr(s)darr + HNO_3(aq)#

#3.# #H_2(g) + Br_2(g) rarr 2HBr(g)#

#4.# #2NaOH(aq) + H_2SO_4(aq) rarr Na_2SO_4(aq) + 2H_2O(l)#