I want to make a bird (about 4 feet tall) so it would have long, thin legs. Would you be able to make an armature for that out of tin foil?

Any long, thin limbs suspended apart from the body should be re-enforced stronger internal supports (wire, rebar, steel, mesh). Especially if an arm or a freely floating lock of hair is unsupported on one end. There is a significant chance of breakage the longer and thinner the limb. 

When re-enforcing a sculpture with an armature, make sure that the PTP encases a large portion of the actual weight bearing supports. There is a reason for this. Ultimately in a few decades, the foil core will turn to dust and this will leave the sculpture unsupported. The armature risks rattling around in a big hollow sculpture without offering any strength.  A good solution is to press groves into the finished foil and lightly secure (hot glue, pins) the armature in these trenches. During the scratch coat, these groves are filled up and the armature is encased in the PTP, properly held well below the final surface.

Pal Tiya Premium is an excellent sculpting material for large scale pieces and can easily be combined with internal armatures to create amazing pieces.  
PTP is primarily used as a hollow shell material. On larger sculptures, an internal supporting structure is needed. This can be either steel or internal cross sections built of PTP. 
Steel: Rod, bar, lathe, stainless or galvanized. Necessary for over human height applications, very large pieces, or sculptures with components that extend significantly off the surface. Seeking advice from an experienced welder is recommended.  Rebar has a tendency to rust if moisture is propagating through the piece. We recommend spray galvanizing the metal as a precaution. Fully cured and sealed PTP sculptures are naturally resistant to 
moisture permeating the material.
PTP cross sections: These need to be 3/4 inch thick and be positioned to transfer any additional weight applied to the shell to the ground. Often a series of interlocked internal cross sections. Mostly for simple, ground based and non-suspended pieces. 
When using steel, the interior armature must connect to the PTP shell. Otherwise, the armature offers no support over time. Foil will disintegrate over decades within the piece, potentially leaving an armature rattling around inside not doing any good.
This connection can be done by directly encapsulating some part of the wire, steel or lathe. If the sculpture is to be climbed on, or suspended over head height, significant portions of the shell must hang on to the metal. This transfers the weight from the shell, through the steel, to the ground.  Connect the shell to enough steel to support this additional weight any place you feel could be compromised; whether it is an arm sticking away from the body, or where a foot hold could be used for climbing.  The armature must be strong enough to support the weight of the entire piece on it's own 

We have a comprehensive video on Armatures for all sculptures here