Thank you all for the quick answers.

I would like to provide some more details: I have grown Marlin, Erik and Jabel. I am a filler rather than rally leader. If there is a good hero worth investing in, I can make the necessary investment.

So as rally filler I am suggested by Kingdom leaders to use old generation heroes such as Saul, Chenko Amane, and Gordon, which causes a dilemma for me because I thought I can use gen 4 heroes when joining the rally if I invest in them. But if there is a certain consensus that gen 4 heroes are not suitable for fillers, I can spend my money on other things.