Create a new column for set of values by multiple conditions

Let's say I have a data table:

> a<-data.table(col1=c(7,85,1905,22,250))
   col1
1:    7
2:   85
3: 1905
4:   22
5:  250

I would like to add a new column in the same data table, considering these conditions condition:

if col1(i) <10 then "A"
else if col1(i) <100 then "B"
else if col1(i) <1000 then "C"
else "D"

So, I'd obtain

   col1 col2
1:    7    A
2:   85    B
3: 1905    D
4:   22    B
5:  250    C

I have tried the ifelse method but it puts "A" in all columns, and doc says that this method works only on first element of the vector it uses.

dt[, col2 := ifelse(col1 < 10,'A','B')]

I don't want to use complex and long loops to do that so if somebody can explain how it works in R, I'd be thankful.

Regards.