B = {Range[5], Range[6, 10], Range[11, 15], 
          Range[16, 20]} ;
TableForm[B]

Part[B, {2, 3}]  //  TableForm

Part[B,  All, {2, 4}]  //  
      TableForm

Part[
        B, {2, 3} , {2, 4}]  //  TableForm

Select[
      B[[2]], # < 10 &]

Select[
      Part[B, All, 2], # < 10 &]

(* 
      Alternative Transpose (//Equivalent to Transpose[
                  matrixIN]//)*) 
TransposeEST[matrixIN_] := 
      Module[
{matrixOUT}, 
numCols  =  Dimensions[matrixIN][[2]]; 
matrixOUT  =  
          Array[mo, numCols]; 
For[i = 1, 
            i <= numCols, i++, 
matrixOUT[[i]] = 
                Part[matrixIN, All, 
                  i];
]
TableForm[
            matrixOUT]; 
Return[
          matrixOUT];
]

TransposeEST[B]  //  TableForm
Transpose[B] // TableForm
TransposeEST[B]  -  Transpose[B]  //  TableForm

TableForm[
      B]

(B  /.  {B[[2]] -> B[[4]], 
            B[[4]] -> B[[2]]})  //  TableForm