When I claimed that there was another Shard involved, it was based on my reading of the statement
"3 of 16 once ruled, but now the Broken One reigns."
My interpretation of this was that there were three Shards on Roshar, all 3 of which were generally benign (or at least natural for Roshar) and operated within their sphere of influence, doing Shard things. Then, just like on Sel, Odium (the Broken One) showed up and began wreaking havoc with their system, resulting in the desolations and the general war that is apparently ongoing.
Part of this is that I don't want Odium to be Ruin part deuce. Ruin was a natural part of Scadriel, even if he and Preservation had to be kept carefully balanced for the world to be amicable. Ruin's power, held by Sazed, is vital to keep Scadriel running. Odium is being built up as something else altogether, as somebody who is personally evil, not just driven to it by his very nature.
Another part of that is that it makes the story much deeper, and gives us many more delicious magic systems.
So I don't have a lot of information to back this idea up, but I haven't seen much against it either. If there are any interviews *ahem* which might shed light on it, I might be willing to read them, though.
The largest evidence I see for 3 shards instead of 4 shards (without reference to interviews) is that there are 3 magic systems on Roshar: Surgebinding, Voidbinding, and the Old Magic. In each of Brandon's books, there has been one magic system for each Shard on the planet. Allomancy came from Preservation, Hemalurgy from Ruin, AonDor from Aona, and Biochroma from Endowment. Presumably the Dakhor magics came from Skai. The only magic system that doesn't fit with this is Feruchemy because it's a bit from Preservation (uses metals in a similar manner as Allomancy) and a bit from Ruin (has diminishing returns like Hemalurgy). Aside from that, it's one magic system for each Shard.
Brandon has said that the Stormlight Archives will have 30 magic systems *depending on how you count*. Here's how I take that statement: for each Shard, there are ten types of magic, one for each of the 10 Essences. Surgebinding relates to the Almighty, and a only a few have been named so far (Windrunners, Stonewardens, Soulcasters, and possibly Dustbringers). I don't recall any names given of Voidbinding (for Odium) or Old Magic (for Cultivation) abilities, but we've also not seen any concrete examples of these.
As for interviews, I'm fairly sure Brandon has said there are 3 shards in play on Roshar, but I can't recall where I saw that. Even without that, I'm fairly sure the the 3 shards x 10 Essences = 30 Magic Systems theory is correct.