"There were only 4" versus "There was only 4"?
Solution 1:
For some people there's has become an invariable word, irrespective of number; but I'm not aware that anybody extends this to there was. (I'm not talking about non-standard dialects in which was is regularly used for plurals: we was etc: that's a different thing).
So, no, I don't think I would regard there was (only) four channels as grammatical in standard dialects, with or without the only.