So in that four year gap between the baptism of Sally in 1804 and Jane in 1808 (in other words, circa 1806)...we are missing daughter number 4!
Unless there was a burial of an infant, but there doesn't appear to be one in Middleton.
Could the mysterious Tamar be the missing daughter of Robert and Mary Grieves?
ADDED The Middleton BTs for 1805, 1806, 1807, are there on FS