Sasori is definitely the fourth strongest Akatsuki Member.
I'm talking about the Akatsuki as they was, so no Juubito and no Perfect Edo Tensei for Orochimaru.
The top 3 are :
1)Tobi (MS Obito)
Those are without A doubt the strongest members of the Akatsuki, non of the other members can beat them.
the number four strongest member is Akasuna no Sasori
first of all for his haters, he let himself die !, he didn't want to hurt his grandmother, so he didn't dodge...
Deidara always said that Sasori is stronger than him, and Deidara always wanted to kill Orochimaru, so u know what it means, they are both below him (not current Oro)
any attack of Sasori's puppets will make U die in 48 hours from poison if you don't have A [B][I]Plot Antidote.
Sasori got his Hiruko armor and his 100 puppets Jutsu, but those are his minor tools, his main tool is the puppet of the Sandaime Kazekage AKA strongest Kazekage ever,
Sasori killed him when he was just 15 years old !, just by that you can understand that Sasori is A badass, this puppet let him use Sandaime Kazekage's KKG, Iron Sand, probably one of the best KKG in NV, Iron Sand is pretty much impossible to counter, the only Akatsuki members that can counter it are the elite 3, Tobi (Kamui), Pein (Shinra Tensei) and Itachi (Susano'o).
Sasori is also can't feel pain, his body is almost like an Edo Tensei zombie, any physical damage will do 0 damage against him, and also Genjutsu are useless against Sasori, Sasori got only one vital point in his body, his heart, only by destroy his heart he will die, Sasori also got tons of weapons inside his puppet body, like A powerful Flamethrower and tons of Blades.
Sasori is also A true immortal, he can't aged at all, he still looks like A 15 years old kid at the age of 35.
The fourth strongest Akatsuki member, and the strongest Sand Shinobi ever.