Combine four columns (multiple columns) vertically into one column

ajl_ahmed 

Two further strategies, one of which is a serious contribution and the other 'tongue in cheek'.  I treat any values that do not lie within a table or (for more static data) a named range as invalid.  In the past I have even written macros to delete such unwanted data entries, recording only the identified data.  

Here, I used a separate table for each column, so that the resulting combined data set could be expressed in the form

= VSTACK(TableA,TableB,TableC,TableD)

Note: VSTACK is only available on the Insider beta channel at the time of writing. It is quite possible to replace the built-in function by a further Lambda function 'VStackλ' and the code for that is already contained within Sergei's formula.

The other technique involved referencing each table as a Thunk (a Lambda function that returns its content if called with a blank parameter string).  I then used VSTACK to combine these into an array

Tablesϑ
= VSTACK(
     Thunkλ(TableA),
     Thunkλ(TableB),
     Thunkλ(TableC),
     Thunkλ(TableD)
  )

Thunkλ
= LAMBDA(x,LAMBDA(x))

The worksheet formula expands the array of Thunks

= REDUCE("Combined", Tablesϑ,
      LAMBDA(acc,Tϑ, VSTACK(acc, Tϑ() ))
  )

This formula works but is distinctly 'over the top'.  I would only use the approach if the tables were distributed in different locations over multiple sheets and I needed to evaluate which Tables were to be used for multiple calculations.