Bottom is a country bumpkin, a laborer, who thinks very highly of himself. In his conceit, he tries to come off as being far more educated than he truly is. As a result, he frequently mixes up words. We see other instances of this when we first meet Bottom in the opening act. For example, the story of the deaths of Pyramus and Thisbe is most definitely a tragedy, yet Bottom refers to it as a "merry," which is a comedy (I.ii.4). An even better example is seen farther down when he is raving about how well he will perform the part of Pyramus saying that he "will move storms" and that he "will condole in some measure" (23). By condole, he actually means "mourn," meaning that he will move the audience to mourn the death of Pyramus.
Hence, we see that mixing up words is a part of Bottom's character and part of what makes him such a fool. Thus, it is no surprise to see him mixing up his verbs in his soliloquy in Act 4, Scene 1. Bottom is actually meaning to quote a passage from the Bible that describes the inadequacies of man's senses:
The eye hath not seen, and the ear hath not heard, neither have entered into the heart of man. (Corinthians 2:9)
Instead, Bottom matches senses with the wrong verbs, such as "The eye of man hath not heard, the ear of man hath not seen" (IV.i.214-215). Other than the fact that he is a foolish, uneducated character, another reason for his verbal confusion can be that he is in shock from having just been a donkey. We see his shock expressed in the lines above these when he stutters and stammers about what he thought he was (a donkey) and what he thought he had (donkey ears), as we see in his lines:
Methought I was--there is no man can tell what dream.
Methought I was, and methought I had, but man is but a patched fool, if he will offer to say what methought I had. (211-213)
This passage is also revealing because he refers to the foolishness of man in his phrase "patched fool," meaning a fool in ragged clothes, like a court jester, which is one of Shakespeare's central themes. Hence, it makes perfect sense that Bottom would, in his state of shock, refer to the foolishness of man and mix up his verbs when trying to quote a biblical verse describing the inadequacies of man's senses. In addition, his verbal mix up shows us just how completely inadequate mans' senses truly are.