Keeping a system consistent definately saves money and frustration in the long run. The OM-4s are readily available, and the 4T's aren't that expensive. The spot meter is a wonderful adjunct. I don't use it very much, but it sure comes in handy at times.
I have to disagree with the comments about a separate spotmeter. I've got a Sekonic that is nearly as big as an OM body, and takes as many batteries. It is very cumbersome to use, and if the lighting is at all variable (scattered clouds), it is practically worthless. There is no comparison to the ease of turning the aperture ring to align the dot to the center, all while keeping the scene composed exactly as you want it. And the OM's are not just a spot meter. They have a multi spot meter that automatically averages the readings. So if you have a problematic scene you can expose it to optimize different regions.