Golang random string collision frequently

I try to generate a random 4-chars HEX string with the following code. Theoretically, the combination should be 2^4*2^4*2^4*2^4 = 2^16 but there are couples of collisions in 1000 rounds. Would like to know if the random seed is not spread evenly or any other possibilities?

package main
import(
  "fmt"
  "math/rand"
  "time"
)

const letterBytes = "abcdef0123456789"
const testRun = 1000
const tokenLength = 4

func main(){  
  rs := make(map[string]int, testRun)
  for i := 0 ; i < testRun ; i++ {
    k := RandStringBytes(tokenLength)
    _, ok := rs[k]
    if ok {    
      fmt.Printf("collision key:%s on round:%d and %d \n", k, i, rs[k])
    }
    rs[k] = i    
    time.Sleep(5 * time.Millisecond)
  }        
}

func RandStringBytes(n int) string {
  rand := *rand.New(rand.NewSource(time.Now().UTC().UnixNano()))
  b := make([]byte, n)
  for i := range b {
    b[i] = letterBytes[rand.Intn(len(letterBytes))]
  }
  return string(b)
}

Result: Try on Go playground

collision key:9536 on round:664 and 328 
collision key:a313 on round:685 and 382 
collision key:2ba6 on round:742 and 91 
collision key:fe63 on round:937 and 146 
collision key:0391 on round:962 and 226