I think it's time to drop that 4th nap really.
As nap 1 is working well you could leave that first A time as it is for now, increase later if/when needed.
Increase second A time to 1hr 45min and you should get a more consistent nap length, be prepared to help through the nap for a few days in case of any bit of OT caused by the A time increase but she should get used to it quite soon.
If you get a good second nap then another 1hr 30 or 1hr 45 before nap 3. If she stays asleep longer than a CN it's okay so long as you wake her ready for an A time before BT.
LO will be more likely to stretch a little longer between E if she is asleep. if she cannot go 4hrs it's ok to get a top up if needed or run E and S times separately so that the A can be long enough to encourage a good nap.
There are no set times or schedules for this, but here is a guidance of what it might look like:
WU 7
E 7
A 1hr 30
S 8.30 - 10.00
E 10
A 1hr 45
S 11.45 - 1.15
E 1.15 (this is only 3hr 15 since previous E, if she is asleep she may not notice the extra 15 min)
A 1hr 45
S 3.00 - 4.30 (this one will reduce to become a CN when the A times increase to 2hrs) If this nap is very short you might need to wing it for the rest of the day to squeeze in another CN, otherwise you go for EBT
E 4.30
A 1hr 45
E 6 the BT feed
EBT 6.15
Whilst it looks a bit off track towards the end of the day bare in mind this is a relatively short period of time and A time will increase to 2hrs which will push BT later again. Over the next 2 months A time will increase more, getting to approx 3hrs at 6months.