Try a Safe Boot from the HD, (holding Shift key down at bootup),Does it still do it?
Have you tried reseating the Graphic card?
Have you done a PRAM reset, CMD+Option+p+r...
Likely that you bumped the graphics card.
Disconnect the monitor and reseat the graphics card (take it out and put it back in).
Removal and replacement of a graphics card is similar to PCI:
Just be sure to release the little green clip on the hook towards the end of the card as you lift the card out (behind the connector slot).
A broken capacitor will give some nice stripes on the monitor.
You don't say what G4 it is that you have.
I have listed all OEM cards and all upgrade possibilities on the following two page links:
Generally speaking, a G4 will use any of the cards listed on the first "Part 1-OS 9" page.
A MDD should stick with the Geforce4 MX or the Radeon 9000 Pro. A Geforce Ti is nice if available.
A Digital Audio or Quicksilver is best off with the same two cards, but can also use any of the others listed.
The Sawtooth or Gigabit Ethernet can use any of the cards as well. The Sawtooth, however, cannot support ADC.
An upgraded card that supports Core image for OS X should be considered.
Any G4 can use a Radeon 9700 Pro or 9800 Pro.
Those two cards are the top cards for the Sawtooth or Gigabit Ethernet.
The DA or QS can use any of the cards listed, but will benefit nicely from a flashed Geforce 6800 GT or a lesser Radeon 9800 XT.
The MDD cannot use a Geforce 6800 GT or Ultra, but can use a Geforce 7800 GS, or a Radeon 9800 XT. A flashed Radeon X800XT (FireGL X3) is also a nice card for the MDD.
Any card considered for purchase MUST be stated as Mac and OS X Compatible or it is likely a PC card that won't work.
eBay is the best source for used cards.
Likely you have what is called a Sawtooth. or possibly a Gigabit Ethernet.
The System report identifier (for example PowerMac3,1) is the best number to go by, with CPU speed helping to ID model.
For Tiger, and plans to go to Leopard, you will want to stay with a Radeon 9800 Pro.
If you are handy with a soldering iron, the capacitor can be resoldered to the board......