For some good tips for anal sex, checkout the following link...
Ten Rules For Anal Sex
First of all, I want to tell you that you should never, ever perform vaginal sex after anal sex without first cleaning off your penis with body soap... As a general rule you should always do vaginal sex first and finish with anal sex... Doing it the reverse way could lead to your girlfriend getting a UTI (Urinary Tract Infection) due to various bacteria found in the anus... Men can get the same kind of infection doing anal sex "bareback" however, many still enjoy bareback sex and just clean well afterwards...
If you are concerned about a UTI for yourself, then use a condom... Using a condom will also prevent againt pregnancy as it is still possible for a "splash conception" to occur if your semen and sperm come into contact with her vaninal area after you pull out... Still there are many, both male and female, who enjoy the feeling of cumming anally and some women can have a great orgasm this way...
Please keep in mind that if you do not use a condom, you should both ensure that you are disease free of any kind as the absorbtive nature of the anal and intestinal tissues allow for an "easier" transmissve quality...
Though if you are both disease free, take care and are gentle with your anal penetration and thrusting and your partner is on birth control... Anal sex can and should be a safe and pleasurable experience for both of you...
I highly recomend the book "The Ultimate Guide To Anal Sex For Women" by Tristan Taormino... She writes from a female perspective with both males and females in mind... It is something that is a very good read for both men and women and does a lot to dispell the multitude of rumors and dangerous misinformation that is out there about anal sex... Dr. Jack Morin also has a good book called "Anal Pleasure And Health: A Guide for Men and Women" and is another great source of qualified research on this subject...
On your question about cumming early... You could have your girlfriend perform oral sex on you first and then after you cum, engage in forplay with her until you are ready to go again... Sex after "reloading" will make you last a lot longer during sex... Either that or masturbate several hours before you meet up with her... And yes, being turned on does lead to cumming early and should be a compliment to your girlfriend...
For your question about a 4 way... Just ensure that if you do that, you are all tested and are "clean" before you do this... Please remember that Herpes is a very bad disease and can be transmitted by oral sex and has no cure... You don't want something like that... Also make sure that you both are comfortable with the extra partners and everyone knows the rules before hand... Otherwise unexpected results can occur for both of you... Remember that you are dealing with other "real" people here and their emotions could and probably will get involved...
Good luck and I hope that helps you some...