Q:

Multiple if blocks or a single if with AND condition

If I need to check multiple conditions, which is the preferred way with respect to performance
if( CND1 && CND2 && CND3 && CND4)
{
}
else
{
}

or
  if(CND1)
{
   if(CND2)
   {
      if(CND3)
      {
         if(CND4)
         {
         }
         else
         {
         }
      }
      else
      {
      }
   }  
else
   {
   }
}
    }

A:

The performance is the same since it will stop checking the arguments once it has found a false one (short-circuiting), so definitely go with the first one. It's one line compared to like, 10. It also means that your else will be much easier to handle.

A:

They will be identical in terms of performance, but the first form is much more readable IMO.
The only situation in which I'd split the conditions up would be if there were sets of logical conditions:
if (x && y) // x and y are related
{
   if (a && b) // a and b are related
   {
   }
}

Although an alternative there which reads nicely (but does evaluate more conditions, admittedly) is:
bool shouldPayBackDebt = x && y;
bool haveEnoughFunds = a && b;
if (shouldPayBackDebt && haveEnoughFunds)
{
    // ...
}

