You could format your variable within the tab number but it will also change the display within the slider.
Another approach could be to use a dual() expression on a field instead of the variable and to play a bit with line-breaks and font-types and the sizing of the object to create an acceptable visualization (the tooltip-feature isn't designed to be formatted or to display arbitrary content). Here an example how it could be created:
load recno() as T autogenerate 15;
= dual(num(T, '0000') & chr(10) & chr(10) & chr(10) & 'this number was chosen: ' & T, T)
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