This has been said many times already on this site, but here goes...
The answer to all “can I do X”-questions in music is: yes, you can do anything!
Really, these questions have just two subquestions that should be considered:
- “Do I want it”? Everything you put in a composition will have a certain effect on the end result. If you decide to put an F♯ in the melody of a piece in F over a D minor chord, nobody can stop you. It just will sound, ahem... Or to take a more relevant example, it's perfectly possible to use parallel fifths, and it won't sound wrong (except perhaps to some overzealous music educators), but it also won't give you quite the full intertwined-harmony sensation that classical composers achieve with their 4-voice counterpoint (which generally avoids parallel octaves and fifths, though even those composers make exceptions when it's musically useful).
In your example, this questions reads “will it sound disruptive if I insert a single ²⁄₄ bar in the midst of my ⁴⁄₄ piece”? To which the answer is, possibly, but since ²⁄₄ is still pretty even, it will probably be barely noticable. In fact, if that extra half-metre fits logically in the melody, then it will likely sound less disruptive than the pause that a full ⁴⁄₄ bar would incur†. So by all means don't shy away from that possibility, just be sure that it actually feels right.
Whether something feels right is best determined by actually playing it, even if just in a much simplified manner.
- “Is it practical”? Some things might theoretically work out great, but it's unrealistic to expect the musicians to be able to accurately render them out. Obviously this includes technically very difficult lines (it's always good to know yourself at least roughly how everything would actually be played, on each instrument), but also parts that are rhythmically or otherwise too hard to get together. And many musicians aren't capable of playing complex metres. But again – ²⁄₄ really isn't that far-out, almost nobody who's musical at all should have trouble with that. As already said, it's not uncommon either.
Is summary: yes, I reckon you should insert a single ²⁄₄ bar in that piece.
†Then again, it can be good to have a bit of space. A rest in the main voice may be an opportunity to give e.g. the cello a bit of a melodic hook, or just to de-clutter everything.