I've noticed that as well. Even though the block does contain quite a few reruns, many of them are new to the channel and they seems to get the best ratings on the block. It's usually Yu-Gi-Oh!, Sonic X and TMNT. I don't know if the fact that reruns perform better than new episodes is too much of a problem. It just looks more surprising to me than anything else.
Besides, as you mentioned before, ratings aren't that important to 4Kids in the long run. It's all about promoting their properties and merchandise connected to the series.
As for the initial question, 4Kids has never been in competition with the three head channels. It would be seriously hard for them to do that since they only have a Saturday morning block while the other channels are twenty-four hours long. Even if they somehow got a weekday block, I don't think that they would be able to compete with Disney, Nick or Cartoon Network. All three of those channels are established channels for children's entertainment. I wouldn't say that 4Kids is a household name like those channels.
In terms of how long 4Kids will remain in the Saturday morning block business, I think that it all depends on how things turn out for them in the future. They're doing a lot of work with Yu-Gi-Oh! and Chaotic and trying to get more merchandise deals. If that stuff like that can work out and they still see their block as a worthy investment, then I could see them at least having another couple of years. Like I said, I don't think that their fate is clearly written in stone yet either.