The phrase "on all fours with" implies that two things are equal in some way. If you're looking for synonyms, there are plenty of options. You could say two things are "in line with," "in agreement with," "matching," or "on equal footing." Other options include "equivalent," "akin to," "comparable to," or "corresponding to." You might also try "parallel with," "consistent with," or "synonymous with." Ultimately, the best synonym will depend on the context and the specific relationship between the two things you're referring to. Experiment with different options until you find the one that best captures what you're trying to say.