I rented in 4s for 2 years, bought in SEH in 2009 so I can shed some light on both.
First the schools since thats usually one of the biggest issues. I would wager a guess that the SEH elementary and middle schools vs the 4s/ del sur schools are close enough that its most likely a wash. The high school that serves SEH is currently not as good as the Poway school system schools. This didnt deter me from SEH mainly bc my only child just turned 3. That gives me 11 years to worry about where my child is going to go to high school.
Next the Mello Roos/ HOA etc – If you dont know, both areas have phases and the initial phases are cheaper on the MR than the current phases. In 4s, south of camino del norte MR are cheaper than to the North. SEH, same idea. Cheapest MR are in the bottom area to the west of the town center and they go up as you go up the hill. Gated subdivisions in both communities have higher HOAs.
Climate – 4s was an oven during the summer but it might have been due to my house’s location (lack of a breeze). Coworkers who rent in del mar would come out and tell us how much of a difference it is. SEH, there is always a breeze it seems. Also, Im highly alergic to pollen and I didnt have a problem this year with pollen. That is the first time in my life. I was miserable in 4s.
Community layout – I liked the parks (swings/slides etc) better in 4s ranch. I havent found a swing in SEH and the slides are so little for the most part. 4s has a really cool park near the newer developments. I like that SEH has a towncenter, with shops, parks, dogpark etc. The towncenter in 4s was a shopping center. SEH to me feels like a separate city in some respects. SEH has ocean views everywhere and I love that I can see the ocean, sunset every day without getting in a car.
Location – Depends on your commute. SEH is worse for me. I could be at work in 13 minutes from 4s so I loved that. del mar track and beach are 20 minutes and there are many ways to get there. So 4s I think is a better location.
Fire danger – equal. Both were evacuated in 2007. The fire went right past 4s on the north side but it came within 100 yds of our home.
Sense of Community – We had some nice neighbors in 4s but not like in SEH. Our subdivision is very close (events for the kids for every holiday/ block parties etc) and I have heard that about a few other subdivisions.
Clearly I like where I live. I liked 4s but I think we would have liked it more if we owned and we bought in a different subdivision. My co-worker is buying at Madeira (house is being built). I liked del sur as well bc of the community pools and parks but the MR are crazy high and the lots/ houses are tiny.
Bottom line, You get more house/ space for your money in SEH. In regards to time to buy, there are plenty of homes available in SEH. If you want recommendations, pm me. I can also give you thoughts on subdivisions in 4s since we looked at many of them. I may add more later and Im sure you will get more input from 4s lovers, del sur lovers and the opposite.