1. Any trustworthy brand
2. 2.5k
3. Matlab simulations, editing of home videos of the children
4. 14-15'

Question is need what is most important , quad core or 8 gb, esepcially as my pricrange get me into gen3 quad cores?

You need to compromise a bit.
Matlab - really depends on the complexity of your simulation and how large is your data set 
home video editing - depends on the tools you use, whether the software support CUDA or OpenCL utilising GPU for processing.
Performance wise, there is not much difference between Ivy Bridge (gen 3) and Haswell (gen 4) CPU.

If you really want i7 quad core with your budget:
Fujitsu
LH532, i7-3632M + Nvidia GT620M@RM2299 [2.1kg], 4GB, 750GB, W8

Lenovo
Z400 touch, i7-3632QM + Nvidia GT740M + (1366x768, touch)@RM2599, 4GB, 1TB, W8; 2 years warranty

Prices are RSP, negotiable.

Video editing - Pinnacle studio can utilise Nvidia CUDA (just make sure you turn on hardware acceleration in option menu).
If you are willing to forgo quad core, there are more choices 
If a model come with 4GB or less RAM standard spec, you could add in more RAM if there is empty RAM slot (4GB ~ RM100, 8GB ~ RM200).

Acer
V5-473PG-54204G50, i5-4200U + Nvidia GT740M +(1366x768;touch)@RM2299, 4GB, 500GB,W8 *no OD

Asus
PRO45VJ-V0047X, i5-3230M + Nvidia GT635M@RM2399[2.4kg], 4GB, 500GB, W7P
K451LB-WX071H, i5-4200U + Nvidia GT740M@RM2199, 4GB, 750GB, W8
S551LB-CJ120H, i5-4200U + Nvidia GT740M@RM2699[2.4kg], 4GB, 750GB, W8, 3-cell

Dell
Inspiron 14R, i5-4200U + Nvidia GT740M@RM1849 [2.32kg], 4GB, 500GB(5400rpm), Linux, 6 cell; W8@RM2049
Inspiron 14R, i7-4500U + Nvidia GT750M (GDDR5) @RM2499[2.32kg], 8GB, 1TB(5400rpm), Ubuntu 12.04 , 6 cell; W8SL@RM2699
Vostro 5470, i5-4200U + Nvidia GT740M@RM2249, 4GB, 500GB(5400rpm), W8, 3-cell
Vostro 5470, i5-4200U + Nvidia GT740M + 32GB mSATA + 500GB(5400rpm) + (1366x768; Touch)@RM2549, 4GB, W8.1, 3-cell

HP
Pavilion 14-N059TX, i5-4200U + Nvidia GT740M@RM1999[2.03kg], 4GB, 750GB, W8

Lenovo
Z400 touch, i5-3230M + Nvidia GT740M + (1366x768, touch)@RM2199, 4GB, 1TB, W8; 2 years warranty
Z510, i5-4200M + Nvidia GT740M@RM2399[2.2kg], 4GB, 1TB, backlit, W8; 2 yrs
Z510, i7-4702MQ + Nvidia GT740M@RM2799[2.2kg], 4GB, 1TB, backlit, W8; 2 yrs (this is the model with Haswell quad core that is closest to your budget)

Toshiba
Satelite L40-AS125XG/B/W, i5-4200U + Nvidia GT740M@RM2299, 2GB, 750GB, W8

Prices are RSP, negotiable.

that i5-750 desktop CPU is quad core (but only 4 threads), while laptop i5 is only duo core but also 4 threads (hyper-threading).
Don't really has the benchmark to compare i5-750 desktop vs a Haswell mobile (say i5-4200U), but my guess is that they are about even 

Matlab (the newer version, and / or if you use the Parallel Computing Toolbox) can take advantage of multi-core very well.

When you observed the CPU max out, did all 4 core max to 100% or just 1 core? Max out for how long?

Then you better go for i7 quad core mobile CPU if you want to shorten the simulation time.

If you prefer Haswell (4th gen) i7, then the cheapest currently:

Lenovo
Y410p, i7-4700MQ + Nvidia GT755M(GDDR5)@RM2899, 4GB, 1TB, DOS, red backlit, 120w psu, swappable ODD, 2 years warranty
Z510, i7-4702MQ + Nvidia GT740M@RM2799[2.2kg], 4GB, 1TB, backlit, W8; 2 yrs

Negotiate, and it should be only slightly above your budget.