The patter is alternate 3^n+1 and 3^n-1.

So following above formula we get:

3^1 + 1  =3+1=4
3^2 - 1   =9-1=8
3^3 + 1  =27+1=28
3^4 - 1   =81-1=80
3^5 + 1  =243+1=244

So last no 3^6-1=729-1=728