Going from 13.0 SEER to 15.0 SEER means you'll save about 15% on your air conditioning bill. Where I live the electric power rate is about
10.5 cent / KWh (all taxes, fees, etc. included). Let's assume yours is the same for this exercise. 4 Tons is 48,000 BTU. At 13 SEER a 48K BTU system will consume approximately (48K/13) 3.7KW. At 15 SEER the power usage would be (48K/15) 3.2KW. So for every hour that the system runs you will save approximately (3.7 - 3.2)= .5KW x $.105 = $.0525, or a lttle over 5 cents an hour. If your A/C is sized correctly it should run about 50% of the time on a 24 hour basis. So .0525 x 12 = .63/day, or about $19.00 a month during peak summer temperatures. Let's say there are five months a year where the A/C runs 50% of the time, the rest of the year it doesn't run a significant amount.Now we consider the opportunity cost on the $1000 incremental cost. If you were to invest it at 3.5% (Emigrant Direct's current money market rate) you'd earn $35 a year. Depending on your tax bracket that might be a $28 net return. Incorporating this into our calculation we get $19*5 months - $28 opportunity cost = $67 net savings/year. If you 'invest' the $1000 to net $67 in savings it will take 14.9 years to recover your 'investment', assuming that electric rates don't go up.
Is it worth it? In most cases, no. YMMV.