Base 4.
So 1s is still 1s.
10s place is 4.
100s place is 4^2 = 16.
1000s place is 4^3 = 64
10000s place is 4^4 = 256, which is larger than 189, so our number will only have 4 digits: 1s, 4s, 16s, and 64s.
How many 64s go into 189? What's the remainder?
How many 16s go into that remainder? What's the remainder now?
How many 4s go into that remainder? What's the remainder now?