I mean, it could be as simple as allowing a prompt to players when they have a reaction ready and can react to whatever is happening (for example fighters won't recieve a reaction while a spell is being cast, only people with counterspell available get it) then they have like a second to press a button - if they press it, they get a menu of what they can do (and as much time as they need to do it) and if they do nothing during that second the action takes place and the reaction isn't used. I would do the same with attacks of opportunity as well (I don't want to stab away at a guy in full plate armor when I know it'll most likely miss and just waste my reaction).
However, I would put in the whole range of reactions like monks catching arrows and uncanny dodge as well, limiting the reactions to 4 abilities seems somewhat arbitrary.
If you're willing to go afk in combat during enemy turn then you won't get to use reactions and you won't slow down play time too much with just a second here and there - I say that's fair.
