How to generate a grammer from this language?
Start with $i+j=k+\ell$. If $i\ge \ell$, say $=\ell+m$, then also $k=j+m$ and we can write strings in the form $a^\ell a^m b^j c^j c^m d^\ell$. Those can be generated by a grammar, you can use the basic toolbox.

Similarly when $i\le \ell$.