Purchased for $100 about 8 years ago and it's a very useful piece of equipment. The built in mics and battery function allow for good quality field recording and having 4 channels makes it an awesome demo-making tool. It is not sufficient to record music on professional level, today's standards require more editing power. Also, there are no XLR inputs for external mics and no built in FXs for guitars and such.
The best feature of DP004 are the built in mics. They sound very natural and if your band sounds good in a room you can just press record and have it. also, TASCAM makes quality products and it's very durable.
The worst feature is how it functions and communicates with a computer. You cannot just connect it via USB and get the files it recorded. First, you need to set ending point on a song, then you need to create a master, which works just like 30 years ago - you press record and play the whole tune again while the thing records the master track. Then you need to export the master track to WAV format which also takes several minutes for an average 4 min song. Only then you can connect it to your comp and see the files. This drives me mad and I cannot stand all the time wasted just to drop the already recorded materials on a computer.