2991
$${\mathtt{2\,991}}{\mathtt{\,\small\textbf+\,}}{\mathtt{1\,992}} = {\mathtt{4\,983}}$$
Start by letting the number be 1000a+100b+10c+d so its reverse is 1000d+100c+10b+a.
Adding these together we get 1001(a+d)+110(b+c). This must equal 4983.
I'll leave you to take the reasoning further!
2991
$${\mathtt{2\,991}}{\mathtt{\,\small\textbf+\,}}{\mathtt{1\,992}} = {\mathtt{4\,983}}$$
Start by letting the number be 1000a+100b+10c+d so its reverse is 1000d+100c+10b+a.
Adding these together we get 1001(a+d)+110(b+c). This must equal 4983.
I'll leave you to take the reasoning further!