I`m using several DC constant voltage PSU`s, including laptop PSU`s - because they are very cheap and often just lay arround. They are all constant voltage. I am in design phase of my project(s) and still use the mentioned PSUs, because I spent all the cash for LEDs, can`t afford nice PSU right now
. I did a little research online and measured a bit too.
First thought is that you will rather want to use some driver (between leds and PSU) to get constant current, because your leds will be fed with as much current as they only take, and will probably fry (at 4 ampere or sooner), or will live very, very short. This is rather important...
2nd: the driver (step-down/buck)will take about ~0,7-0,75V for his own usage. So you will have ~18,25V left for your disposal. I would definetely go for a "I" regulated (potentiometer or pins) driver, it is much more flexible and it will allow you to regulate the current to the level, where your LEDs eat less voltage than it is disposable.
I use some 31V HP printer PSUs, which are pretty much over of needed voltage of my setup (24V). It looks like that the difference of voltage is toasted (6V) somewhere, I see that the unit draws so much power, as there would be 30V worth LEDs applied. So the more gap beetween voltage of the PSU and voltage drop of your leds, the more power is lost. I don`t think that this is in the driver (alone!), probably the laptop PSU are not designed for less voltage, and allways eat at their V specs.
4th: efficiency of your laptop PSU will be best above 50% of its max current design, preferabely above 70%. So if you will constat drive (with a buck driver) your leds at 1A, then it will be around 23% of max output of your PSU. In this area your PSU will be as low as (very roughly) 40-60 % efficient. Modern laptop PSU`s are about 80-90% efficient at about 90% of their max current output. So for better efficiency I would take a smaller PSU or I would attach more LEDs