look...considering that 4 years experience don't really make you a senior engineer yet, and I assume you are not chartered nor have master degree, as mechanical engineer i think that you shall have something between 28,5K and 36k. And remember that contract shall clearly state that overtime will be paid as per Qatari Labor Law (look for it in internet, overtime will be paid 25% and 50% more), since it is very likely that you WILL to work overtime ;)