You can use Tasker to change mobile connection modes. A toggle I'm not so sure about: I don't think it would be as simple as 4G on/off, as that isn't one of the Network Type settings (Action > Net > Mobile Network Type). Probably you want something that could change it between "2G And 3G" and "Auto" - as both of those settings exist it's probably doable, but would need a bit of thought (I've not created or used a Tasker widget in years myself). A pair of widgets would be very simple, but not what you want I'm sure.
A shortcut to the mobile data settings is probably easier. It looks like the "Any Cut" app I used to use, which allowed you to create a shortcut to literally anything, is no longer available, but a search for it in the Play Store does give you a list of alternative shortcut creators. It should be easy to use one of those to create a shortcut to network settings, to get all the way into the network mode may be less straightforward or require some extra knowledge (e.g. exactly what that particular setting is called internally).