I just started the campaign through MW4 Mercs and thus far I love the way the game play and mission selection is availabnle. It seems much more like real warfare to me then the guerilla style game that you had in in Vengeance.

ANyhow I went with the Kell Hounds as my sponsor, but was wondering what real differences the four sponsors introduce?

I know some start out with more funds or different mechs, but wasn't sure how useful that is.

Secondly, as I started out with the Kell Hounds I made it through the first mission on eaton fairly easily, but I wanted to get one of my warriors out of that flea. I decided to buy an uller to replace it, but wasn't sure if that was the best choice in hind sight.

What mechs would you replace/buy first if available early in the campaign, and how early do you start adding pilots for a second lance?

Right away for the second lance...then start having pilots take turns until they can all go full-time. It's nice to have pilots that have some degree of improved skill instead of having a lance of vets and a lance of rookies.

As far as mechs go, I keep the standard sponsor-provided set until I either get some decent chassis as salvage (that early Puma, for instance) or get enough C-bills to buy a few Bushwackers.

The only mechs that are really worth keeping for a while (in my opinion) that you start with are the Wolfies, the Uller and the Raven. Anything else you can quite safely get rid of when you have enough to get / managed to salvage a halfway decent replacement.

As to the next mech to purchase, WoD is quite correct, make it a Bushwacker or if you can hold out long enough (or it may even be cheaper than the Bushie but available a couple of missions later) get a Shadowcat. Don't bother buying Argus's, they're plentiful in salvage. The last mission on Eaton will probably give you 2 good heavy mechs (Black Knight and the ever hated, by me anyway owing to it's minimal armour and for a Clan mech poor tonnage capacity, Loki).

I am way beyond the beginning game now

But I basically took what I had the first mission and then I think I bought a second osiris for the second mission then i used salvage from there on the bandit mission and things rolled from there.

Never forget your fire support, either. A handful of well-configured LRM boats crewed by your highest scoring pilots make a massive difference the first time you field them. If you have or have access to a 'mech with a large number of missile/omni ports, don't hesitate to use it (and often). Catapults are good, Vultures are better...once you get your hands on a Longbow or two, you can effectively forget about ever dealing with enemy short-range attacks again. AIs just can't deal with effective fire support.

I never really liked the Vultures in Vengeance.. I prefered the Cat... and thats what I did when the second lance opened up i hired mustang and bullseye to do firesupport

The Vulture is slightly better for several reasons...not the least of which is that the Catapult's racks are huge and make easy targets, which translates to quite a few lost arms (and LRM-20 racks) on higher difficulties. The Vulture's racks are torso-mounted and quite a bit smaller (though just as spacious) than those of the Cat, which means fewer lost missile racks that need to be replaced between missions. Plus, you can shoehorn in a 5th LRM-20 if you really want to, which almost makes it a better option than the Longbow later on (simply because the Vulture is so much smaller and, again, less likely to take lots of hits). That 5 ton difference really isn't quite so big when you consider that the Catapult will likely cost you a lot more in lost parts and repairs over time...the only things it really has going for it are BAP and Jump Jets, neither of which the AI uses very well.

hrm lrms in the torsos is not something I had givem much thought to. That is very succintly put there wrath.