It's the geometric sequence with common ratio r = 3.
[tex]a_1=4\\a_2=3a_1\to a_2=3(4)=12\\a_3=3a_2\to a_3=3(12)=36\\a_4=3a_3\to a_4=3(36)=108\\a_5=3a_4\to a_5=3(108)=324\\a_6=3a_5\to a_6=3(324)=972\\\vdots[/tex]
The formula of a general term of a geometric sequence:
[tex]a_n=a_1r^{n-1}[/tex]
[tex]a_n=4(3)^{n-1}=4(3)^n(3)^{-1}=4(3)^n\left(\dfrac{1}{3}\right)=\dfrac{4}{3}(3)^n[/tex]
