The boat/trailer is about 4k but does that include fuel and other provisions you may have on board? What size boat and what kind of trailer? I know mine is listed at about 6700 dry but that includes a single base engine - mine on the trailer with fuel is about 10,500lbs. Just keep in mind the manufacturers listed weight will most likely not match up with what you actually have. I've seen folks around here struggle with just jet skis around here - especially if it is busy and the ramps are wet or have had grass and debris spread about.

For the 4wd question - the Explorer is listed as being 4wd but it is permanently engaged. The **** is to change the settings for the type of terrain you may encounter. Take a look at page 230 in the Owners Manual:

The 4WD system is always active and requires no driver input. It is
capable of handling all road conditions, including street and highway
driving as well as off-road and winter driving. The driver can optimize
more 4WD control by moving the terrain management switch for the
correct terrain. See the Terrain Management chapter for more
information.
Note: When an 4WD system fault is present, the warning Check 4WD
will display in the message center. The 4WD system is not functioning
correctly and will default to front-wheel drive. When this warning is
displayed, have your vehicle serviced at an authorized dealer.
Note: The 4WD Off message may also be displayed in the message
center if the 4WD system has overheated and defaulted to front-wheel
drive. This condition may occur if the vehicle was operated in extreme
conditions with excessive wheel slip, such as deep sand. To resume
normal 4WD function as soon as possible, stop the vehicle in a safe
location and stop the engine for at least 10 minutes. After the engine has
been restarted and the 4WD system has adequately cooled, the 4WD Off
message will turn off and normal 4WD function will return. The 4WD Off
message will turn off when the system cools and normal 4WD function
returns.