Can Kate automatically convert spaces/tabs?
Try to do Ctrl + A to select all text and then Ctrl + X to cut it and Ctrl + V to paste it back. It will automatically format the indentation accordingly to your settings in Kate configuration.
Select the text you want to modify, then 'Tools > Clean Indentation'.
Use key combination CTRL+R to open the replacement dialog window. Now use mouse and mark one tabulator, press CTRL+C and paste it under "Find" section. Under section "Replace" just input four spaces and click "Replace all".