You should have two serial numbers: one for LR3 and an upgrade one for LR4. If you cannot locate your LR4 serial then you need to find that first, options will vary depending in how you purchased it.
There's no need to install LR3 prior to installing the LR4 upgrade. You can just install LR4 and net both serial numbers.
LR4 is asking for your LR4 serial number. Your LR3 serial number will not work. Enter your LR4 serial number and the LR4 install will complete.