I was in a nearly identical situation when I applied! I graduated in 03 and hadn't had close relationships with any of my UG profs, though I had 3 supervisers from my RA job who were willing to write me letters. To make things worse, I didn't even major in psych, and was an unimpressive student, so you're probably in a better position than I was. This is the advice that I got from several different people: even if you graduated a few years a go, it's still really important to have at least 1 letter from UG. If you don't, it may raise a red flag that you don't have anyone who can vouch for you from that time period. In the absolute worse case scenario (this happened to a colleague of mine the first time he applied) programs may wonder if you're hiding something. Others may disagree, but like I said, I got this advice from researchers who have been helping RAs through this process for years.
The good news is that your UG prof doesn't have to write anything amazing, since you already have 3 people who know you well and will write you excellent letters! What I did was write to my UG adviser and explain that I already had LORs from people who knew me currently, but that I was looking for an UG LOR to round out my application. I acknowledged that I hadn't been a memorable student, but asked if she would feel comfortable writing a very basic letter to confirm that a. I took her class and b. I did well in it (I sent her my transcript to back this up). That's all I needed, so she agreed to write the letter. If you decide to do this, I recommend that you also send your UG prof your CV and personal statement - as it turned out, my prof told me that she actually ended up writing me a better letter based on the strength of those materials.
Again, everyone's experience will be different, and others may have very different advice. I'm just telling you what worked for me. Based on what you've told us, it sounds like you've got a very competitive application and will be a strong candidate when you apply. Best of luck to you!