Ambulate

hunkering down onto all 4s from a 5 foot lead in you gain 20 extra movement speed (for 15 total). this 5 foot lead in cannot be changed so watch out in narrow spaces.  you may use a bonus action to return to bipedal mode.  hunkering down is a free action. can stack with other movement effects like Expeditious Retreat, Longstrider or Jump. 

this bonus movement is still granted even if you are currently suffering from movement restrictions, though maybe not while  restricted/grappled by a creature larger than you, or by a creature that cannot move (plants *cough cough*) 

this is great for exploration but there are uses in combat as well, its up to you to figure them out though

for example a human has 30 movement, ambulate gives them 50 - 5 = 45 total 

any creature that has 2+ limbs can use ambulate so long as at least one limb is a leg of some kind and one limb is an arm of some kind