I don't think we know that... do we? Remember, Trish was probably created somewhere around the time of DMC1, since that's when Mundus sent her to Dante. Could've sent her sooner, but he didn't for some reason. Trish was around the same age as Dante, it seems... and he was 28 then.Everything seems fine with one exception: Trish does not age. She looks exactly Eva looked when she was killed, so Eva was killed in her 30's.
I don't know what you mean by ''she looks exactly like Eva looked when she was killed''... Eva was killed when Dante was 8, and there's no mention of Trish already existing at that time.
In any case, Lady still looks like she's thirty to me in DMC4, which would mean she's aged ten years, which would mean DMC4 indeed takes place around ten years after DMC3.