TChunkType = array [0..3] of Char;

  TPNGChunkHeader = packed record
    Length: Cardinal;
    ChunkType: TChunkType;
  end;

...

  TPNGReader = class
  private
    ...
    FHeader: TPNGChunkHeader;

...

function TPNGReader.IsChunk(ChunkType: TChunkType): Boolean;
const
  Mask = not $20202020;
begin
  Result := (Cardinal(FHeader.ChunkType) and Mask) = (Cardinal(ChunkType) and Mask); //<-- Hier bleibt der Compiler stehen
end;