James is right on track, just be sure to use a rip saw for rip cuts and a crosscut saw for crosscuts. The exception to this is for fine tooth bench saws where a rip toothed saw can be used very effectively for crosscuts.. The only things I would add is to try your saw on a long cut in a piece of scrap first in order to detect any tendencies to lead off the line before you try it on actual project stock, and correct the saw if necessary. Also, when it comes to trimming, it becomes a judgment call by the craftsman as to whether it is better to saw or plane the excess off. If I only had a quarter inch or so that had to come off of a rip I believe I would just reach for a roughing plane to do the work. You do have to be more careful with a crosscut because of the inherent weakness of short grain and the increased difficulty of planning end grain. On crosscuts you should pre-score the grain, leave less stock that requires planning, and use a shooting board and/or backing strips to prevent blowout during end grain planning.
On the subject of a choice between #4 or #5, I think a #5 can be more versatile if you take the time to set it up properly between different uses (rough work and smoothing) but it all gets back to the type of work you intend to do the most. For me, I believe that a beginner should buy a good Stanley for each of the basic plane functions and set them up and tune them for the individual functions. As inexpensive as these older planes are it is money well spent to keep from having to make the changes necessary to support the different functions. For the cost of one of the newer planes you can easily buy a full set of the old Stanleys from the local flea markets or estate sales and have some great tools in the bargain. There are some good brands other than Stanley but a beginner would have difficulty sorting the good stuff from the junk.
Above all else, have fun.