hi, i'm wondering if u can help me with an optimisation problem..?

A company produces two types of cattle feed, which consist entirely of alfalfa and wheat. Feed 1, which sells for $3.00/kg, must contain at least 80% alfalfa
and feed 2, which sells for $2.60/kg, must contain at least 60% wheat.
If the company can buy up to 1000 kg of alfalfa at $1.00/kg and up to 800 kg of wheat at $0.80/kg, what should it do to maximize its profit ? [Suggestion: let feed 1 be produced by combining x1 kg of alfalfa and x2 kg of wheat and let feed 2 be
produced by combining x3 kg of alfalfa and x4 kg of wheat.]

alfalfa: 0.8x1 + 0.4x3 =< 1000
wheat: 0.2x2 + 0.6 x4 =< 800

want to max:
z=3.00-[(0.8 x 1.00)x1 + (0.2 x 0.8)x2] + 2.60-[(0.4 x 1.00)x3 + (0.6 x 0.8)x4]
z= 2.4x1 - 0.48x2 + 1.04 x3 -1.248x4

I'm pretty sure this is wrong because I tried in on matlab and only got a result for x1. Do you know where I have made an error? or is there an easier way to solve this? many thanx