Are we going to be adding more category's to seasonal? We are heading into season 5 soon...

I believe it is because there haven't been any new seasonal runs since then. Basically no one submitted any season 3 runs. We are heading into season 4 not 5 btw. Possibly more interest again now that there have been some significant changes. I think a lot of people were just burnt out.

How do I submit a run for season 4 since there's no category for it?

Season 4 is open for submission now

Keep in mind - rules are still being hammered out with the item changes. So please be patient with any review/approval.

plan for tempering? currently can temper legendary patterns onto lvl 1 rares, seems like an oversight by blizz but I could be wrong

Rule for now I believe will be tempering at level 15 (when the quest opens up). I think blizzard is just making it more accessible to people. They never responded to me when I asked if tempering/imprinting was intentionally made available immediately to new characters.

Crimson has been implementing no tempering until lv 15 just on the assumption that it will be added to the rules. That sounds pretty good in all fairness, and he was able to set all his new times this way...

I've done the same on my runs (if they are ever approved by someone else). I have not heard any disagreements for tempering be set to 15 in the rules discussion in the D4 speedrunnig discord. The official rules will likely be updated soon.

At this point, for campaign, I'm pretty sure tempering before level 15 would slow the run anyway -- the only fights it'd matter for are astaroth and airidah, and even there it'd only save ~30 seconds on astaroth (and no time on airidah) -- between the teleport time and the temper itself, I think tempering in campaign runs before level 15 may be a moot point.

That said, I think the rule should be level 15 anyway, it makes the most sense to have elixirs/tempering/gems/occultist at the places they'd be for a first-time playthrough anyway.