This
webpage will generate relatively easy puzzles. You can choose the cells that will have givens, but NOT specify the givens. It will attempt to create a puzzle using givens ONLY in the cells selected. It will not always succeed, even when the mask of givens is known to be one that will hold a good puzzle.
This
webpage will also allow you to pick the cells for givens in a different way. It will allow you to specify what these givens are, though you cannot simple select a cell and type in a '4'. Each time you click a cell, a random value selected from those that can still be placed in that cell is shown. Just keep clicking until you get what you want. It will then generate a puzzle using these givens, adding as many additional givens so that the puzzle has a unique solution.