Though I think it's impossible to guarantee a single unique solution, I can think of a couple of things that can certainly help. I'll use your "One rhyme, / Space, time" riddle as an example...
Firstly, as GentlePurpleRain mentions, you can try to directly limit solutions by including rules within the riddle. For example:
I am one word. I am one rhyme,
I am all of space and time.
With this minor alteration, you're forcing the answer to be a single word. Also note that by simply adding the "I am" to the "one rhyme" line, you have effectively restricted things further, invalidating the incorrect answers you received, since they relied on a broader interpretation, such as "I have one rhyme" (innuendo), or "I describe one rhyme" (relative/dimension).
Second key to keeping a riddle short while limiting the possible answers is to try and include seeming contradictions. Look for various forms of wordplay/metaphor/etc based on your intended answer, and then pick clues such that they seem to point in opposite directions. This will naturally limit answers, as a solution for one clue will likely break another. For example:
Split: So small, I'm but one rhyme.
Whole: Boundless, in space and time.
Maybe not the best example of what I'm trying to get at, but here I'm trying to create a contradiction where the answer is apparently something very small, yet very big.
I'm not saying either of my examples above are fool-proof (or even necessarily an improvement on your original, which I quite liked by the way), but hopefully they illustrate my points.
One final note... Everything you do to your riddle is going to involve a compromise. Obviously my examples above have made your riddle longer, but unfortunately that's generally a side effect of making it more restricted. The key is to choose carefully when to compromise in balancing the brevity of the riddle with its clarity, because your compromises will provide diminishing returns. That is to say, going from a one line riddle to a two line riddle won't necessarily make it less appealing, but it can go a long way to making it less ambiguous. On the flip side, going from 5 stanzas to 10 stanzas probably isn't the most efficient way of reducing ambiguity, but it sure makes it more unwieldy and less appealing as a riddle.
Basically, I think the goal of creating an unambiguous four word riddle is a noble, but incredibly difficult goal. Let me know when you succeed. :)