4 out of 15 isn’t bad. As for other ratings that really depends on your goals? If you have aspirations to be a professional then yes at some point you’ll need to multi-engine rating. If you’re purely doing it for recreation then it’s a matter of how far and how fast do you want to fly? Twins are generally faster and have great range but there are some pretty slick singles with some speed and range as well. Twins are nice for flying over water as well because it’s always nice to have an extra engine rather than go swimming. Again it really comes down to what you’re trying to accomplish.
If you are going to do this professionally then you’ll need your Commercial license and then you’ll need to start building time. To fly for an airline you’ll need 1500hrs and the most common means of building that time is flight instructing. To do that you’ll need your CFIs as well.
For my budget I am going to a flight school with one instructor. This school 30k cheaper than another one in my area. He is always on my schedule mostly which is nice. Our goal is to finish my ratings in 2 years. What is the most efficient way to get hired and into the airlines?
2 years to finish your ratings? The most efficient way to get to the airlines is to attend an accelerated flight training program. You could have all of your ratings AND 1500 hours in 2 years.
I’m never one to tell anyone what to do but if you’re looking for efficiency going to the local flight school with one instructor generally isn’t it. That said this is America and you can do as you like. You say you want to get your PPL and IR by the end of the year. That’s totally doable provided you remain consistent. My suggestion is you make that a HARD goal. That’s 3.5mos from now and you’ve already started. If you don’t have both by then you need to seriously and honestly assess what you’re doing. No excuses. No “it was the wx, the plane, my schedule, the instructors schedule” etc etc. If you’re serious about becoming an airline pilot you should be serious about your training.