as you say, 4 light years to the closest star... how fast can we actually propel something with today's technology? It's a ridiculously low % of C as i understand it...then we have to slow it down, so as not to destroy our precious payload in a high-speed impact, or flyby.
The way i see it, without an absolute breakthrough... the only thing I can imagine feasible are purely mechanical probes.
When traveling at such ridiculously low percentages of C, it'll take many thousands of years to visit all but the closest stars--likely even that kind of time for our closest star. Whats the life expectancy of a million dollar o-ring? How long could a bank of computers be designed to operate for before too many units fail from too many cosmic ray hits?
Given today's technology, or even increasing speeds by 10x what we can do with today's technology, I'm having a hard time imagining anything realistic.
If we ever got to the point of creating effective Von Neumann probes (and i think we will) that could make serious exploration, perhaps colonization, possible, across a many thousand year time span.
If we ever got to a point where Terraforming makes something habitable, then perhaps few batches of test-tube baby deployments could be considered. Anything less than a terraformed world, and I'd call it cruel to stick someone unwillingly inside of a can for their whole lives.