Solution

In $G_2$, productions are:

$S\rightarrow aSb\mid bSa\mid SS\mid \lambda$

In $G_4$, productions are:

$S\rightarrow aSb\mid bSa\mid SS\mid SSS\mid \lambda$

The only extra production in $G_4$ is:

$S\rightarrow SSS$

But this production is already possible using $S\rightarrow SS$ again and again.

For example:

$S\rightarrow SS\rightarrow SSS$

So, $SSS$ does not generate anything extra.

Hence, $G_2$ and $G_4$ generate the same language.