No OS 4 device can be upgraded to OS 5. OS 5 only runs on ARM-based processors. The problem is the underlying hardware, not anyone's upgrade policy.
Sony hasn't offered an OS upgrade since the N710 conversion, and there's no reason to expect this to change now that Sony has left the US and European markets.
Palm has announced no OS upgrades to any recent device. OS 6.0 requires 16 MB Flash, so only the TE, T3, and T5 have the hardware to load it. All these devices have the horsepower to run OS 6, but PalmOne hasn't made their intentions known on this issue.
You can ascertain if an upgrade is available for your OS 3.x or 4.x device by going to the the manufacturer's support website and looking for the support files for your device. If you have a Sony, don't waste your time.