If you have restored the laptop back to factory settings and tested the RAM in other machines then it can only be a compatibility problem between the RAM and the motherboard, perhaps in the timings. When you upgraded to 4GB from 1GB did you add two identical 2Gb modules or one single 4Gb? If the former were they both from the same supplier, the same make and model number? Have you tried with just one 2Gb module, one at a time to see if it just one of them that is causing the sleep problem? Any chance you can identify memory certified by Acer to work in your model and then buy that?
Fujitsu Lifebook P727 laptop review