The newer devices stream 4k and lower. You don't need a 4k TV to use the newer Roku devices. (In case you were under that presumption.) Many of the older Roku device models are still in the stores and some are even in the clearance shelves. (Walmart, Best Buy, Target, etc). Some people in this forum said they found them at Dollar Tree even. Here is a good reference page that lists the various Rokus models and years released with the various features.
Some say to be be a little more careful purchasing online as sometimes the products are misadvertised, either honestly or maybe dishonestly in some cases. Basically, verify you are getting exactly what you want.
Also may want to consider that older devices will eventually get phased out and may not be as readily updateable as the newer devices, especially with the increasing number of streaming channels being created and released. Buy too old a device and may not be able to install a Channel you may want in the future. Last release of the Streaming Stick appears to be 2019 so don't really see that as an issue. But again, always have to consider what your budget allows.
Roku Community Streaming Expert Just another Roku user... I am not a Roku employee. Insignia RokuTV, Ultra 4660, Premiere+ 3921, Express 4k+ 3941, Streambar 9102
As @AvsGunnar indicated, a Roku capable of 4K also supports HD resolutions of 720 and 1080. My 4K Streaming Stick+ runs quite nicely on my 10-year-old Samsung 1080p set.
I do make a point of setting the Roku's "Display type" to match my set's actual capabilities rather than leaving in on the default "Auto detect". This seems to alleviate occasional problems when slipping in and out of commercials which are presented in a different resolution than the program they are embedded in.
Roku Community Streaming Expert I am not a Roku employee, only a user like you. Please, no support questions via private message -- post them publicly to the Community where others may benefit as well. If this post solves your problem please help others find this answer by clicking "Accept as Solution.".
Streaming Stick 4K+ 3821 | Streaming Stick+ 3810 | TCL Roku TV 43S245/C107X