You may be interested in my own experience with elderly B&W speakers - in my case DM3s, now 35 or 36 years old. These still sound very good and remain in constant use in my living room. However, several years ago I also purchased a pair of some much smaller AVI Pro-Nine Plus speakers, which I use in the kitchen, and so I have in effect been running a comparative test between the two for quite a while.
Once one is settled in the lounge with the DM3s, they sound fine - but moving between the two rooms one is always struck by how much better whatever one is listening to sounds on the AVIs in the kitchen. I am in no doubt that the latter are very much better - they present music with a greater clarity and reveal detail that simply does not catch the ear on the DM3s, and sound very much more lifelike (we attend a lot of concerts so know what instruments really sound like). We have occasionally set the AVIs on top of the B&Ws for direct comparison, and each time we do so I resolve to replace the B&Ws with a pair of AVI floorstanders as a matter of urgency, so marked is the improvement in the sound of the more modern speakers. But of course, that would mean spending a lot of money, and, as I say, when one is not actively comparing the two the old DM3s do in fact continue to sound extremely good. So I continue to use them and will probably do so for some while.
But I am in no doubt: in the case of the DM3s (and I am guessing that this may well be true of your DM4s), replacement with a smaller speaker could represent a genuine improvement, so long as you choose carefully - and I would certainly have no hesitation in recommending that you try the Pro-Nine Plus before you make a final decision.