Well now that the lion guard season 3 has ended, we see why there can't be a 4th season of the lion guard because kion and his friends are no longer part of the lion guard. 

They are now part of the night pride defending the tree of life. So Disney could make a new series revolving around the events and challenges kion and his friends face in their new duty. It would also be nice to see more of kion and rani's relationship, plus they are king and queen now. It would be better not to go to the next generation because kion and rani are not even grown up, kion doesn't even have half of his mane. It would be nice to see a new series called something like 'The Night Pride' or 'The Tree of Life'. I am sure Disney can easily make up more ideas for the series.

If a season 4 for the lion guard was made it would probably be about vitani's guard but let's be honest most people would be more interested in watching kion, rani and their friends more as they grow.

I personally am a big fan of the lion guard and would love to see more of the lion king world. I think the show will have a lot of support.... I hope 'cause then again I can't predict what others think. So please please if you agree with me and my idea sign this petition and hopefully Disney will hear our call! <3