Let\textquotesingle s define the symbol $$\Phi~ $$ as an operation follows the rules beneath:  ($$1$$) If $$a ~\textless{} ~7$$, then $$a\Phi b=a+b\times b$$.  ($$2$$) If $$a\geqslant 7$$, then $$a\Phi b=a-b$$．  What is the value of $$4\Phi \left( 8\Phi 3 \right)$$? 
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