As good as the idea sounds, I do not think Disney will make a 4th season, for numerous reasons.
The first is that Ford Riley, on Instagram, said that Disney was satisfied with the ending of the 3rd season, so the show didn't get a continuation.
The second reason is because Ford Riley no longer works for the Walt Disney Company as a whole. The final season was written and finished ahead of time, so by the time that Battle for the Pride Lands and the entire first half of the season was leaked, Ford Riley's contract with Disney ended with the show. Most spin-offs or renewals are made with the same team as the original, so unless someone takes over in Ford Riley's place, we might not see any new TLG content come out.
And just a side-note, I seriously think we should stop asking him anything TLG related. It's been a year and people are constantly spamming "Where is TLG S4" on his posts, even going so far as to go onto his old posts from 2016 and beg him for S4. In no way does that sit right with me, and I think we should stop looking at this man and instantly viewing him as the "God" of Disney's The Lion Guard.
The third reason is in regards to Mercury Filmworks. Around the same time that the finale of The Lion Guard aired, Tangled: The Series, another show animated by Mercury Filmworks, jump-started its final season, which lasted from October, to March. When you link the time-lines, it's easy to assume Disney canceled the Lion Guard in favor of Tangled: The Series, especially when you compare the run-times of their third seasons.
But I digress, even though Tangled is over as of a year ago, Mercury Filmworks isn't just gonna up and return to TLG now that it's canceled. Especially since now in these times where a certain entity which I will not mention is damaging the film industry as a whole. Unless another animation studio will take over for the show, the chances of TLG being renewed for the 4th season are highly slim.
4thly, and this was only in the wake of recent events, bear with me, Disney is planning a sequel to their CGI remake, which ironically premiered on the same day that TLG S3 finished production. So Disney isn't exactly abandoning Simba and his lions, but the thing is, the remake does not fit into the original's canon. That's why it's referred to as a remake. So anything that is added into its continuity can't be considered canon to the original, and vice-versa.
And get this - They announce this sequel ONE YEAR after their remake film comes out. One year. It took 2 years for the production of Simba's Pride to be finalized, it probably took between 2-4 years for the pilot of TLG to be completed, and it was like, 6 years in between the two Frozen films. And remember, the Lion Guard ended not even a year after the remake had been released. But Disney waited one year to announce a sequel to their remake, proving that they held it in high regard.
To some fans, this may indicate that Disney isn't abandoning the franchise and could possibly make a 4th season. I say that isn't true.
If Disney truly cared about The Lion Guard, and its status, they would've either -
A) Renewed it for the 4th season.
B) Announced a new series that's a continuation of TLG
C) Announced a new film that serves as a sequel to both TLK 2 and TLG.
However, that obviously wasn't the case here. In the case with A, it normally takes a year for a series to be renewed for another season. TLG was announced to have a third season before the second season finished its run. Yet here we are almost a year and a half into the future and no signs of any official TLG continuations.
With the case of B and C, I stand by the fact that Disney preferred their remake to have a sequel rather than their TV Show to have a sequel, and they did this one year after their remake and TV Show finale was released. So Disney does not prioritize their series enough to make a continuation.
Finally, and this is based on pure theory - It is unlikely Disney will ever make a sequel to Simba's Pride. They had multiple attempts and they didn't pull through with it. 1 1/2, instead of being a true prequel, or a true sequel, was a midquel. When TLG was announced, many people assumed it would pick up after Simba's Pride, but instead it went with the classic midquel route, and it wasn't until the very last episode where we finally get a post-TLK 2 story. They would've made a post-TLK 2 film years ago but they didn't, and it has been 2 decades since it came out. If Disney made another TLG series or renewed it for the 4th season, it might be meant that they had to base the story after the second movie,
If a 4th season does come out, that would be cool, (and I do hope they have better animation mange to retcon the continuity errors that the first 3 seasons brought in the show) but if Disney was at all planning another continuation of the original film, we would've known for sure. Instead we have no proof at all.
I think for those who consider TLG canon, then we might as well just consider Return to the Pride Lands to be the finale of The Lion King franchise.