Generally speaking, a 5-pound, 4-month-old cat is on schedule for a normal growth pattern, considering size can vary among breeds and genetic traits.
At four months of age, kittens continue to grow extremely fast, and their weights fall between 3 to 5 pounds. A kitten within this weight range will still be slender but remain playful and energetic. Here is a general breakdown of size and weight:
Weight
For a 4-month-old kitten, the average is 3.5 to 5 pounds. Thus, if your kitten weighs 5 pounds, this means that it's the highest end of the average, which is perfectly normal. Bigger breeds, such as Maine Coons, may be heavier than that at this age, but that's normal, and their siblings can be lighter.
Body Size
The body of your kitten is still lengthening at this age, and they are likely to have a narrow, youthful frame. Their coat might also still be soft and fluffy, though they'll start developing their adult features as they approach 6 months.
Growth Rate
Kittens typically double their weight at 4 weeks, and at 8 weeks, they'll be about 2 pounds. Past this point, they also start to gain weight at a steady pace. By the time they are about 5-6 months old, they may reach up to about 5-7 pounds, and most adults reach their size within one year.
That said, a 5-pound, 4-month-old kitten is considered a very good weight, and its growth is probably right on target. It is very important to monitor their diet for a balanced kitten food at this stage of their rapid growth. Of course, a visit to the vet is always advisable regarding monitoring its growth rate.