I love catapults, but i will say the C4 is my least favorite.. It just is hard to run as a boat, at least for me anyway.. I much prefer a C1, with dual LRM15's and some energy.. But outside of that... Seeing you have the C4..
Step one
Drop 2 JJ's and swap out your two SML's, for ML's and add an extra ton of ammo
Step two, Get Double heat sinks.. first upgrade
Step three, Get endo steel.... and you can swap to a pair of Medium pulse lasers. for more close range punch. I much prefer them over ML's especially when i only have 2 on a long range mech.
step 4, Get an XL engine, Either an XL-300, Or XL-280.. though honestly when i run my C4, i use an XL-260.. I figure it is my slowest of my long range cats, might as well pack a punch.
once you got this, add artemis, as you will now have the tonage..
BUT, remember cat's dont always have to be LRM boats.. You could run 4 SRM's with some ML's or MPL's and be a short ranged brawler back up.. aka follow the assaults and when the brawling starts do some crazy up close and personal damage.
Just going step 1-3 should set you up in a decent place where you can grind and grind, and make money to get your next mechs, and XL engine.. (i say get the XL-280 as it is more flexible in many other mech builds, especially when learning the game)
But, If you are having enough fun grinding C-bills in your modified C4,, It might be worth it to keep grinding till you can afford a Cataphract 3D, as it comes with an XL-280, and is one great mech! then you can try your hands at a heavier, more brawler heavy.. Before you decide to sink your life into grinding 2 more Catapult chassis. That said, When you do unlock elites, and finally get extra master module slot, that is when your mechs truly shine.
the Jaeger, hero firebrand, and the cataphract ilya are MC mechs that come with an XL-280.. If one happens to come on sale, and you don't mind spending some MC.. this is a great way to get a XL engine. You can also click here to look for other mechs that have those engines.. You can get an XL-300, or an XL-275 in many mechs
http://mwo.smurfy-net.de/
While the XL-275 and XL280 weigh the same, the XL-280 is the better choice for a catapult, but other mechs cap out at 275, and the increase in speed is very small... So as the most versatile engine, the XL-275 nudges ahead.. You can grab one of the 55 ton mediums, SHadow hawk, Griffin or WOlverine, along with the hero heavy metal, and i think something else to get that engine..
While you may have to grind double, you can think of it this way, you have a great mech to play later.. Sometimes it is better for a new player to get a few different mechs, and customize them, and play around to see what suits them. Though nothing wrong with playing trials too.
Good luck!!!!!
PS, using LRM's doesnt mean you have to sit in the back.. I find often in my LRM mechs, i tend to skirmish in the 225-450 range depending on my back up weapon set up.. at say 300m,, an LRM 40 SHOT packs a huge punch...