If you get the ER4, I suggest getting the P/S adapter as well. ER4S is significantly better sounding that ER4P, as long as your source is reasonably powerful.
Both DBA-02 and ER4P are great. ER4S is a notch above both in sound quality IMO with the right source. DBA-02 is all about musicality and immersion. It is a very fun sounding IEM, but still very accurate overall. The way it combines musicality and technical ability in one package is quite amazing. It has everything you will need in an IEM - tight, punchy bass, clear, natural mids, detailed, well extended highs, good accuracy and precision, great detail, excellent imaging and separation and a great sense of space. ER4P is a different beast. It is not as fun as DBA-02 overall, but to my ears it does surpass the DBA in accuracy and often sounds more realistic than the DBA. ER4P has the tightest, most accurate bass I've ever heard in a headphone. It is modest on impact, but goes very, very deep and is extremely tight and defined. DBA-02 sounds a bit sloppy by comparison, although if you just listen to the DBA, you won't notice it much - it only becomes really apparent when AB'ing the two. ER4P also has the best timbre I've heard in an IEM - all real instruments and vocals sound realistic and sometimes I even get a vivid sensation that a singer is right beside me due to the great accuracy. The highs on ER4P sound subdued compared to DBA-02 treble, but they are not recessed. Rather, I think that it's the DBA-02 that's a little boosted in the treble, while ER4P is just neutral. Detail resolution in the treble is slightly higher on DBA-02, while bass and especially the midrange is more resolved on the ER4P. The differences are generally small however and it will come down to personal preference which IEM you enjoy more. Clarity is a bit better on DBA, but then again, the difference is small. Dynamics and stereo separation are a bit better on ER4P. Soundstage is more 3D on DBA-02, but ER4 just presents what's on the recording and doesn't add extra depth to the sound. Personally, I prefer ER4P for critical listening, for instrumental music, and for well recorded electronic music due to the higher quality bass. DBA-02 is more fun for pop, rock and bass heavy genres like rap, dance, trance, hip-hop, but ER4P still sounds more accurate with these genres in my opinion. It just doesn't have the exciting grit/aggression of the DBA-02.
ER4S surpasses DBA-02 in most aspects I think. DBA-02 beats ER4P in clarity and sound depth, but ER4S is stronger in these areas than the P and matches the DBA-02. Moreover, ER4S also adds extra transparency to the already great ER4P mids and tightens up the already tight ER4P bass. The only issue I have with ER4S is that it can sound overly bright and a bit lean on the bottom end at times if the source is too weak to drive it properly. With a good source, it should be a nice upgrade over DBA-02, although with a different sound signature and presentation. For unamped use, it's a toss up between DBA-02 and ER4P. I described the differences above, so just pick your poison based on your preferences.