Hi, since you're lo is only coming up 4mths she will only be just moving from a 3 to 4hr EASY, and the activity time gradually increases with age. I didn't start to stretch activity time until after 4mths cause my dd just wasn't able to stay awake without getting overtired. But by 5mths she was doing the 2hrs activity time easily. BTW, those guides are more based on a baby that naps for 2hrs at a time, my dd didn't, so she took a little longer to adjust.
For now, justs follow your own baby's cues and it will all work out ok. There is quite a difference from a 3mth old and 6mth old, that long activity time is something to strive for by 6mths rather than being able to do it from 3mths iykwim?
HTH