You should be able to use a standard PC monitor with you A4000 with the aid of a 15 to 23 pin adaptor. These adaptors came as standard with the A4000 so there is a good chance you may get one with the A4000 you bought. I would ask the seller if this is the case. If not, most reputable Amiga dealers should be able to locate one of these adaptors easily enough. You could also try eBay for one as well.
If the A4000 is not configured to display a VGA screenmode already then you will first need to adjust your display preferences with either a 15KHz monitor or a RF adaptor, such as the A520, plugged into a television as most PC monitors will usually not display anything below 31KHz.
Once you have set up a suitable screenmode using the 'DblPAL' or 'DblNTSC' monitor drivers, then you can plug in the PC monitor to see if things work. The 'VGAOnly' driver will probably be needed in DEVS:Monitors also as sometimes the 'Dbl' modes may not give a high enough sync rate. You may have to swap between the 15KHz display and 31KHz display adjusting the preferences until you find a mode that the PC monitor likes.
On a final note, be careful when swapping the displays about feeding 31KHz signals into a 15KHz display like a TV as it may cause harm to the 15KHz display. Having said that, I have used a 1084 monitor to set up VGA modes on my A4000 and all it did was display out-of-sync garbage at the top of the screen without any harm being done.
Best of luck.
