A lease complicates your decision somewhat. Have you received a payoff number on the lease so you are sure that your net loss is the 4k? The payoff will probably be your remaining payments plus the residual plus any fees that might apply. Also, since your miles driven are increasing to 2,000 per month, a factor in your decision should be the overage charge under the lease for excess miles if you keep the MDX. You need to estimate that number in your calculations.
California is one of the few states that do not reduce the purchase price of a car by the value of the trade-in before computing sales tax.
2011 MDX Advance, Aspen White Pearl/Umber
2013 Lexus ES300h
2000 Chevy Silverado 2500