The glass temperature (point at which it starts becoming molten) for PVC is 82°C (179°F). The generally accepted maximum operating temperature for PVC is 70°C (158°F) though again, this can vary by product. Pressure ratings are at room temperature (21°C/70°F), and they go down as the temperature goes up: at 70°C the pressure rating is decreased by around 20% (eg, schedule 80 PVC rated for 230psi is only rated for 43psi at 70°C).
(Note: CPVC, which is more commonly found used for potable water applications, has a glass temperature of 106°C (222°F) and unlike PVC, it can also be used for hot water supplies. Since you said drain pipe, it's probably regular PVC.)
The exhaust temperature of a dryer typically cycles from 50 to 71°C (120 to 160°F), though there is a potential if the dryer is broken it could be higher (any dryer made since the 1980's has a thermal fuse in it, hard to find info but it sounds like these often blow at 198 or 185°F).
So worst case, is if the pipe was a pressurized PVC water pipe, and you have really high water pressure (90psi -- typical is 30 to 60psi), and your dryer is broken and operates near 198°F for a while before the safety fuse shuts it off: depending on how enclosed this area is, it may actually get the pipe hot enough to weaken it enough that it bursts from pressure.
However, drains are not pressurized, and so in the same situation, the temperature is still not high enough to get to the glass temperature, and from 4" away, it's definitely not going to transfer that much heat to the PVC (unless you had it enclosed in a small insulated space with no air flow at all). I'd say you are pretty safe.
According to Chicago Building Code
18-28-504.1 Installation.
Clothes dryers shall be exhausted in accordance with the
manufacturer’s instructions. Dryer exhaust systems shall be
independent of all other systems and shall convey the moisture and any
products of combustion to the outside of the building.
Exception: Where the make and model of an electric dryer is known
and the manufacturer’s installation instructions do not require
exhaust, see Article 4, Ventilation for room exhaust requirements.
Venting a dryer inside is a potential fire and mold hazard, and can negatively affect indoor air quality causing sickness (and/or death, maybe).
You should bring this up with the condo association, the building owner, and potentially the building inspector. A possible down side to this action is, the condo association could ban the installation of dryers altogether. So you'll want to try and suggest some alternatives.
I'm going to guess, the condo association is worried about the appearance of vents sicking out all over the building. A potential appeasement would be to select a nice looking vent (voted on by the condo association), and force anybody who installs a dryer to use the same vent.
They may also want all vents to be installed in similar locations, so the vents are uniformly placed on the building. A good way to insure this would be to install all the vents at once, including installing vents for units without dryers (in which case they should be sealed until in use).
Best Answer
Portable probably means electric, so yes that should be safe in the short term.
You may soon have mold problems and the like from excess moisture, if you keep doing this, but there's no immediate danger from the exhaust, as would be the case with a gas dryer which exhausts combustion products.
You could simply put clothes on hangers and set them to dry - during the winter, they dry fairly quickly just hanging on the shower curtain rod over the tub, without blowing lint around the house.