Re-watching season four, can you please tell me why Beth decided when she saw all that evidence to take down Nick and not Rio because her and Rio had been at each other‘s throat‘s all the time and she wasn’t happy that he took over the strip club so what made her decide to take Nick down and protect Rio? 
Yeah! It's because of the events of 4.14 and 4.15 that she really switches allegiances, but it's actually part of a much slower and more incremental development of her shifting feelings forwards Rio across the whole of season 4.
It starts with her burgeoning guilt over using him in 4.04 when she sets him up to handle Dave, then develops with her stress dream about betraying him and her subsequent inaction to find any incriminating evidence in 4.06, her offering him an out seconds before believing she's setting him up in 4.07, and her glee at finding an alternate in Gene before choosing Annie and Ruby over him in 4.08. By 4.10, she subtly chooses him over Dean when she asks Dean to go instead of Rio when he catches them at Paper Porcupine, and significantly, in 4.12, Annie basically tells her that Rio's incapable of harming her, something which doesn't have immediate effects but which certainly weighs on Beth and likely majorly informs her interpretation of his actions of returning her furniture and money in 4.14.
When that happens, Beth has to consider that Rio kept her furniture, which likely meant he always intended to give it back—meaning that even at the height of his anger and desire for vengeance, he seemed to see a different sort of future for them. After Annie came back unscathed from her kidnapping, too, it reframes Rio's ability to harm her or the people close to her. Basically, she sees more clearly that he's not the threat that she once thought he was.
On top of that, Rio, prompted by Nick insinuating he is going to steal Beth away from him, pivots his tactics and becomes much more direct and honest about his feelings for her. She's touched by the return of the money as his signaling that he supports her and believes in her ability to pull off the impossible (a stark contrast to Nick and Dean in the same episode), and when he also conveys that he's listening to her in 4.15 and when he openly decides to align himself with her over Nick—even choosing to come talk to the cops with her—she puts her trust in him and chooses to protect him when the time comes for it.
I talk about the incremental shifts a bit more in depth here.









