Assuming you have US passports and your bags are cleared to MLE then KARFA is right , landside is best

Clear immigration and follow the signs to "local buses"

Take the 482 or 490, it's free just tell the driver you are going to T4

Once you get there (10-15 mins) then go upstairs and clear security as normal

The trains are all a pain as you have to take two

A taxi would be pretty p*ssed about a terminal transfer so I probably wouldn't do that unless desperate for time and willing to tip big!