SR4 doesn't have the ability to make vehicles from scratch, unlike rigger 3. What you're going to want to do is take a vehicle around the right size and capability - and put arms, legs, and looks-like-a-human mod on it.
For comparison, the Akiyama, a childsize drone has a body of 4. The Otomo, a metahuman replica cyborg has a body of six. The tomino, a troll-sized combat deathrobot has a body of ten. So that gives you a ballpark for what to shoot for.
Full Mechanical Arm - 4000, 2 slots(facility)
Walker Mode - Bodyx500, 2 slots(shop) (Overmodded. Penalty: Metahuman movement rates. 15/30/90)
Rigger Adaptation: 2500, 1 slots(Kit! hah.)(Overmod if you can. Seriously, if a pokeball sized microdrone can have it for free, so should a car or bike)
OR cyborg Adaptation(facility), your AI probably doesn't need to pay for the CCU, though, but the cyborg adaptation technically doesn't require it.
Touch Sensors: Bodyx500, 1 slot.(shop). (Again, overmod if you can. Its pretty much the one last common cyborg traid/mod)
Maybe Pimped Ride(looks like a human) or Mimic System to get the human looks.
Overmodify as much of that as your GM will let you to get a finished human-like walker drone. You'll need 4-8 slots, depending on how lenient your Gm is. Slap that all on a cheap vehicle with about the right body. I like motorcycles for that, because you can adjust the body with Sidecars to basically add mass and get what you want, but you could use a drone for the free rigger adaptation that comes with all drones.
Now add stuff to the cyberlimb capacity on your cyberlimbs. Armor to taste, and a commlink, and you're good to go.
Here's an example, taking advantage of the contrails extra modslots, using the standard limit of availability 12
Thundercloud Contrail: 5000
Handling: 1 Accel: 20/40 Speed: 180 Pilot 1 Body 6, armor 4, sensor 1.
Walker mode: 3000. Half speec/accel, handling +1
Touch Sensor: 3000. +1 sensor/perception. +1 handling.
Full Mechanical Limb 4000. Cyberlimb Armor Enhancement 2 (600, Avail10, 4 capacity each), smartlink: 1000(avail10, 3 capacity)
Full Mechanical Limb 4000. Cyberlimb Armor Enhancement 2 x2(600, Avail10, 4 capacity each), Commlink 2000+commlink(2 capacity)
Rigger Adaptation: 2500.
Mimic Modification 1: 5000.(like a human dude. Now you even have an excuse to share meals with the team)
Adjusted Stats:
Handling: 1(3) Accel: 10/20 Speed: 90 Pilot 1 Body 6, armor 4(
, sensor 1(Mess around with custom sensor loadouts yourself! Its a Vehicle. Enjoy signal5/capacity12)
Slots used:2+1+2+2+1+20=10, 0 remaining
Total cost: 30700+commlink
And thats roughly how it does. A slightly more elegant solution to the commlink problem involves upgrading the vehicle with a new Response chip and System/Firewall to go with it, possibly using the Modular Electronics mod from 4th anniversary's changelog.