append values from the second column to the end of the first column Python

Question

I have a data file (.csv) that looks like below: (only values)

| First Column | Second Column | Third Column | Fourth Column |
---------------------------------------------------------------
| Yes          |            20 |              |          0.35 |
| No           |             6 | happy        |          4.01 |
| Yes          |            13 | Okay         |           3.1 |
|              |             2 |              |             1 |
| No           |             9 | Hello world  |           0.5 |
| Yes          |            50 | Puppies      |               |

Now I want to append the data values from the second, third, and the fourth columns to the first column so that the final output would look like this: (Basically just to stack up the values from each column. Note that there are NULLs and they should be kept. )

| First Column |
----------------
| Yes          |
| No           | 
| Yes          |
|              |
| No           |  
| Yes          | 
|           20 | 
|            6 | 
|           13 | 
|            2 | 
|            9 | 
|           50 |
|              |
| happy        |
| Okay         |
|              |
| Hello world  |
| Puppies      |        
|         0.35 |
|         4.01 |
|          3.1 |
|            1 |
|          0.5 |
|              |

I'd like to write in Python with Pandas data frame in iteration because the data file has a few hundred of columns. My initial thoughts on the logic are:

1. Count total number of columns N.
2. Count total number of rows R.
3. Copy R rows of values from the [a + 1]-th column, where a is 1 initially and increments by 1.
4. Append the copied values to the end of the first column.
5. Iterate this [N - 1] times.
6. Drop all columns except the first column.

If you can help me with the core coding part or if you have any better suggestions, it would be greatly appreciated.