The Amiga 4000 has two screen-refresh-rates, 15 KHz and 31 KHz. If you want to watch both modes you want to:
1. Get a scandoubler so you could use any regular pecee monitor to view all screens.
2. Get a true multisync/bi-sync monitor like the commodore 1940/1942. There are also some monitors called Microvitec 1438 that are 15/30KHz compatible.
You could also use workbench in 30KHz mode (by using VGAOnly montor driver), but most games use 15KHz, and furthermore you won't be able to see anything until WB is fully loaded. Beware that most multisync monitors, more than 99%, can't view 15 KHz modes.
-edit-
And any decent TV or monitor can be used by buying an approprite RGB->Scart cable. There are lots available on Ebay. But you'll miss 30KHz modes that give a sharper picture in higher(AGA) resolutions.