Read

Hi all, I am trying to read a messy data  but facing  difficulty.  The
data has several columns separated by blank space(s).  Each column
value may have different lengths across the rows.   The first
row(header) has four columns. However, each row may not have the four
column values.  For instance, the first data row has only the first
two column values. The fourth data row has the first and last column
values, the second and the third column values are missing for this
row..  How do I read this data set correctly? Here is my sample data
set, output and desired output.   To make it clear to each data point
I have added the row and column numbers. I cannot use fixed width
format reading because each row  may have different length for  a
given column.

dat<-read.table(text="x1  x2  x3 x4
1 B22
2         C33
322 B22      D34
4                 D44
51         D53
60 D62            ",header=T, fill=T,na.strings=c("","NA"))

Output
      x1  x2     x3     x4
1   1     B12 <NA> NA
2   2    C23 <NA>  NA
3 322  B32  D34   NA
4   4   D44  <NA>  NA
5  51 D53  <NA>   NA
6  60 D62  <NA>  NA


Desired output
   x1   x2     x3       x4
1   1    B22    <NA>   NA
2   2   <NA>  C33     NA
3 322  B32    NA      D34
4   4   <NA>   NA      D44
5  51  <NA>  D53     NA
6  60   D62   <NA>   NA

Thank you,