Ok first of all you check it your RAM. How much MHZ Speed. It is variable it will give problem.
So you can put your old 512mb on your slot after that run your laptop. Any problem will come or not you check it. Any problem is not there. Please check your new RAMs.
Your laptop will probably have problems with 4GB of RAM to be honest, I think the most your model can have is 1gb in each slot, not 100% tho. There are also issues with a 32bit 0S and more that 3GB of ram as (google it).
So now I’ll understand the laptop will not turn on even with the old ram? well I’m guessing an orange light is flashing on the front? This can happen for a variety of reasons, like if you or a piece of metal, like the bit from the RAM ‘compartment touches a stick while the laptop is on. You may have shorted out another component, try taking all the ram out, and then trying it. You may have to check individual components.
Toshiba laptops are picky on RAM (as are others). You need the correct speed and type of RAM as not all RAM is created equal. Check the RAM vendor’s web page to see if they support the Toshiba model you have. Even mainstream RAM manufactures may not support your model with every type of RAM they produce.
With Windows XP being a 32-bit OS, you will not be able to use the full 4 GB of RAM, but will have somewhere between 3 GB and 3.3 GB of usable RAM. This is an address space limitation of the OS. Using a 64-bit version of the OS would give you access to the full amount of RAM, but has its own issues.
As to the system not booting after replacing the original RAM back in the system, reseat the RAM. Check that it has seated completely and that the catches engage. That is the number one failure I see on replacing RAM in a system.
Check if there are any flashing failure codes or if there are any beep codes.