I have a siemens 4000 handset and I like to

Yes. All DECT compatible handsets will register to a DECT compatible base unit.
If you have the manual for either phone, just follow the instructions.
If you dont, some manufacturers do have some odd quirks to the registration process, but most follow the following.
Look on the bottom of the base unit - there should be a 4 digit number somewhere. There will probably be lots of other longer numbers and bar codes also, but these are unrelated.
The base unit needs to be put in to registartion mode - usually done by either switching it off, waiting about 30 seconds, and then switching it on. Or, press and hold the base unit's page / find handset key. NB. the base unit stays in registartioon mode for about a minute, so if you dont complete the handset steps before this time, you have to start again.
Somewhere in the handsets menu will be a register handset option. On selecting this it will then ask for the 4 digit number above. Enter this, and after a short wait, the handset will be registered. Remember, this has to be done before the base unit times out.
Also note; base units can only register 4 or 5 handsets (depends on manufacturer), so if you've already got this number, you'll have to de-register one of the others first.