Not sure what to tell you then. Everything you could possibly need to know about the memory can be gotten from the system. The BIOS can tell you quite a bit, but CPU-Z will retrieve everything.
Type, Voltage, Capacity, Speed, Timings. Nothing else should be required.
If you want someone to tell you exactly what to buy, you will have to figure out exactly what you have. There is a method in front of you.
Crucial's memory selector is a good resource for figuring out what an OEM machine will take, but it won't necessarily give you an exact match to memory you already have.
Or, take the memory out and bring it with you, hand it to the sales rep and say you want a 4GB stick of this.