When ever the player presses the 4 key in the creative menu it replaces the item in the forth hand with a block or an item and the original item in my forth hand gets deleted

This Works as Intended. When hovering over an item or block in the creative inventory then pressing a number that item/block goes into that hand slot.

WAI. If you hover over something in creative or survival and click a number, it swaps the thing you are hovering over with the thing that is already in that slot. In the creative menu, this means that the thing in that slot is deleted.

Thank you for your report!
However, this issue is Working as Intended.

The report you have submitted is working as intended, mechanics of the game may change between updates.
Things such as graphics, sounds, world creation, biomes, redstone, villagers, and animals may not work the same in current versions.

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