My optiplex turns on and the 3 and 4 light stay on and it beeps 4 times and then 2 times but it does not come to the logon screen

James,
It sounds like the refresh rate is set to high. Since you do not mention what OS is installed I have listed information on how to change in Windows,XP.
Start the computer and press the F8 key. Choose Safe Mode. Once the system gets into Windows follow the steps below and then restart the computer. If this does not resolve your issue email me at my email address below and include your OS information.

How to Change the Screen Refresh Rate of Your Monitor

1. Click Start, click Control Panel, click Appearance and Themes, and then click Display.
   
   Alternatively, right-click the desktop, and then click Properties.
2. Click the Settings tab, and then click Advanced.
3. Click the Monitor tab, and then click the screen refresh rate that you want to use.

NOTE:

• The default refresh rate setting is 60 Hertz (HZ); however, your monitor may support a higher setting. Check your manufacturer's documentation for information about the settings that your monitor supports.
• If you use a higher refresh rate, you can reduce the rate that your screen flickers; however, if you specify a refresh rate that is too high for your monitor, your screen may become unusable and your hardware may be damaged.
• Changes to the refresh rate affect all users that log on to the computer.

Or your system is giving off a beep code in which case the beeps come in a 3 beep sequence. Below is the beep codes and what each represents.Beep Codes

Troubleshooting steps and more information about the OptiPlex 320 beep codes Beep Codes Code Cause 1-1-2 Microprocessor register failure 1-1-3 NVRAM 1-1-4 ROM BIOS checksum failure 1-2-1 Programmable interval timer 1-2-2 DMA initialization failure 1-2-3 DMA page register read/write failure 1-3-1 through 2-4-4 DIMMs not being properly identified or used 3-1-1 Slave DMA register failure 3-1-2 Master DMA register failure 3-1-3 Master interrupt mask register failure 3-1-4 Slave interrupt mask register failure 3-2-2 Interrupt vector loading failure 3-2-4 Keyboard Controller Test failure 3-3-1 NVRAM power loss 3-3-2 NVRAM configuration 3-3-4 Video Memory Test failure 3-4-1 Screen initialization failure 3-4-2 Screen retrace failure 3-4-3 Search for video ROM failure 4-2-1 No time tick 4-2-2 Shutdown failure 4-2-3 Gate A20 failure 4-2-4 Unexpected interrupt in protected mode 4-3-1 Memory failure above address 0FFFFh 4-3-3 Timer-chip counter 2 failure 4-3-4 Time-of-day clock stopped 4-4-1 Serial or parallel port test failure 4-4-2 Failure to decompress code to shadowed memory 4-4-3 Math coprocessor test failure 4-4-4 Cache test failure
Thank You,
Dell-Jesse L
Dell Social Media and Communities
email [email protected]

1-3-1 through 2-4-4
Memory not being properly identified or used


http://support.dell.com/support/edocs/systems/opgx620/en/ug/tools00.htm#wp1120374

the 6 beep codes will be in a specific timing i.e. two beeps, a space 2 more beeps, and another space and 2 more beeps.

Looks like it may be a memory problem according to the lights.

Possible memory failure

Reseat all DIMMs and restart the system to retest.

Here is the link to the beep codes and diagnostic lights for your dell optiplex gx 150 HERE

The Beep codes are in Table 2 and the diagnostic lights are in Table 3.

The first issue you had with one long beeps and two short beeps is a memory module error. see the following HP website:
http://h10025.www1.hp.com/ewfrf/wc/document?docname=c01732674&cc=us&lc=en&dlc=en&product=1842155 I would replace the memory chips and try again. If you have more than one chip try booting with one at a time to discover that one of them may be bad.

Test DRAM refresh

Read this very carefully. Turn off your ps3 from the back. Then turn it back on wait unit the light is red. Turn on the system through the front keep holding the power on. You will hear a beep one time that just tell you that the ps3 is powering on keep holding the button until you hear the second beep this will rest your video setting. If that dosnt work call me at sony