Regarding the preview on Prime 4 - Jason (Jwill) has said that they are working on that feature, but that there are some challenges due to having to figure out how to route the preview to a channel that is not in use on the internal sound card.
Reese can probably elaborate, and maybe even with a link to the comment from Jason (Jwill), which can be found here on the forum. 
No problem if it only works on the working channels.
You also have effects settings to improve… on the P4.
Still amazing devices I love to use
If it only works on the working channel? Its intended to work on the channels where the faders are at 0.
But to my understanding, the issue getting it to work on the Prime 4, is caused by they have to figure out getting the OS to route it to a channel that is not in use… and since you in 4 channel mode have to press to select deck when loading a track, this also inflicts on the preview.
And they want to ensure that the preview output doesnt end up mixed in the Master volume.
Once again, this is just my understanding of the work in progress (and Reese probably could explain it way better) - but its not as easy as we might think.
Hahahaha I got that!!! Clever put! 
I must say, I find it on my SC5000’s to be very convenient to quickly check before loading. Even when 4 decks are available.
…but you can quickly check before playing. Why the need to check before even loading?
It’s like saying “I’ll just listen to this 12in before putting on my turntable” or “I’ll just listen to this CD before putting in the CD player”. 
Guess so but…I don’t know, there are shitload of really great and unique feature request here that are “ignored” so it doesn’t really help when they add a feature that is kinda missing from the beginning. It all seems so far like playing catch-up instead of taking the lead.
I mean, you cannot get any other impression when you realize you can make playlist on the unit but cannot add star rating to tracks, right?
And I must say, I will forever defend the right to tell somebody to RTFM if they did not took the effort to do it - if there is a minimum requirement for DJ-ing then that is to read the manual for your gear.
I said “it’s convenient”. If you start using it, it becomes clear that it’s just a simple solution to a problem that I didn’t had. 
I understand your comparison, but we’re not in those times anymore.
I wonder if denon are holding off any more on-player editing of database contents like the stars, the hot cue and loop names etc becasuse of how easy it is to corrupt the database.
If James Hype switched from Pioneer to Denon DJ (with the Prime 4) it would surely use all 4 channels simultaneously. 
But if this happens, I doubt very much that he would take a Prime 4: he would probably make an X1850 + 4 x SC6000M 
When a new user, or any user asks a question on the “Forum”, why is it anyone’s business as to the need or use of the feature/request??
If it is popular or you agree with it, upvote or if you don’t care, doesn’t apply to you, why can’t you just move on like normal people on a “discussion forum”.
This Forum is way out of control with self-righteous know it all’s telling well meaning people what they should like or not.
Coded language or not, it’s easy to troll or hide behind a keyboard when you have nothing to lose and call people idiots. Just Pop out of the woodwork even on a “Audio” forum??
Talk about trolling…this is the real virus in society.
Personally, I like the preview where and when I can get it, if not feasible or possible, just say that and it’s a done deal. If it’s in print by Denon somewhere…someone will ask about it or inquire about a timeline. If it didn’t come from Denon and is just speculation, then that answers the question.
Track preview is much faster than load the tracks to deck to listen to them. Think a play button as on new prime engine software. No drag and drop to listen just play press the play button located to the song name.
True… but there is a difference in a discussion or a rant that’s more suitable in a taylor swift’s youtube music video comment section no? 
(ohhh the drama sometimes )