Once again, pharmacists don't really know this stuff -- all they are trained to do is write prescriptions. While they do know some contraindications, I've been down this route too many times and they usually will tell you to contact your doctor. Most psychiatrists don't even know how to safely start and stop these medications. The only way to learn this stuff is to read about it, both by the mainstream pharmaceutical company paid studies and by the outliers, and then decide who you believe. Pharmacists are great if you think you've taken an overdose or are taking two meds and want to know if you can take them together, but they do not get trained on how to taper off psychotropic meds. That's theoretically what psychiatrists study, but my experience over many years is that you have to find a good one to get good answers on this. Pharmacists are just very careful not to give advice that is closer to therapeutic info than what's on the info that comes with every prescription.
great question for your pharmacist - seriously, just give them a call
Second that. It should go easily, as you haven't been on Effexor very long, but this along with Paxil is the most difficult med to stop taking. Always better to quit this one if you're going to sooner rather than later. So to be safe I'd taper off before starting the Celexa so you know if any reactions you get are withdrawal from the Effexor rather than start-up side effects of the Celexa. But again, after only 4 weeks, this should be easy.
I would suggest that you wean yourself off of the effector because sometimes just stopping hate creates more issues and you have to give time for the celexa to get in your system. Hope that helps :)