Like Caution says, it's most likely OK. Those wall wart adapters are not regulated anyway and I would not be the least bit surprised if you check the voltage with a meter and find that it is actually something more.... like 6 or even 7 volts! That's because with no load, voltage will be high. With a load, voltage will drop. However, if you are concerned, you can install a diode in series and it will drop the voltage probably right around where you need it. That is due to the nature of the diode which typically has a forward voltage drop of about 0.6 volts. Shottkey diodes have slightly lower voltage drops ranging from 0.15v to 0.46v. Using a standard diode of sufficient amp carrying capacity, your 5v adapter should theoretically then output 4.4v to the device. However, once again, since these adapters are non regulated, pin pointing down the exact voltage to your device will be difficult.
Here is another bit for you to chew on.... most devices use a power adapter specifically designed for that particular device based on it's expected current consumption. Since they are almost always non-regulated on older devices, the voltage rating is "average" based on the expected current consumption of the device in normal use. Like mentioned above, once there is a load, the voltage will sag. If you get a weaker PS adapter, then the voltage may sag MORE than expected and would result in a lower than normal "average" voltage from that adapter and inversely, if you get a larger very robust PS adapter that can output gobs of current, your marantz might not draw enough current to cause the voltage to sag to the "average" value and therefore, it will get a steady stream of higher than normal voltage. How can you check? Simple... just monitor voltage with adapter connected and device operating.
Now, with all that being said... if you are concerned with your Marantz and really don't wish to damage it.... you can simply build your own power adapter using batteries. Yep, get a standard 3-D cell battery holder, wire up the output pigtails to the correct plug and if your marantz uses 3 AA batteries internally, you'll find that with external D cells, you will get at least 5 times the life you did with the AA batteries. Plus you can be assured that you have provided the proper voltage to your device. Although D cells doesn't last forever like a plug in adapter would, on a small portable like that marantz, it will last an obscenely long time.