The St. Croix combo is one of the best values on the market. Redington also offers a great valued combo in the RedStart series.
A 4 wt rod will handle trout up to about 4 or 5 pounds so this line size won't be your limiting factor for the streams you are going to fish. However, the best way to make a rod wt choice is by the size and weight of the flies you are going to use. If you are not going to fish flies bigger than a #16 (maybe a #14) the 4 wt rod will do just fine. If you are going to fish flies up to a #10, the 5 wt is the better choice. If you are going to fish flies up to #6 (#4 in a pinch), a 6 wt is the better choice.
In other words, instead of making a rod choice on the basis of the size fish you are going to fish for, make it on the basis of the size and weight of flies you are going to fish. Remember a heavier line rod can always fish small flies; however, a light line rod doesn't do too well with larger flies.