First and foremost, if you can spend 7 or 15 dollars, do it, and save the MC till you need to buy a bay as you add mechs down the road. The 15 dollar amount is the way to go as you can add 10 bays and that is not even with a sale, so earn one more with earned MC at events, or maybe even a weekend event, and now you can master 4 more mechs at 3 variants each, counting the extra bay you have atm
If you are liking them and it sounds like you are, i'd just keep playing and saving C-bills, till you really get an itch to try something new. But HBK's are so solid, and so flexible, you can try PPC set ups, AC5's, AC10's, gauss, Large lasers, Pulse.. you get the idea.. there is really no need to even bother picking up chassis to try out any of these set ups. So if you are wanting a new weapon load out, spend a bit of money and try out some new systems. For the cost of one mech with upgrades you can basically pick up a lotta weapons instead, and play a whole new mech, at least in feel anyway.
My IS account i sorta non-stopped just kept grinding chassis for a long time. I ended up playing Parts warrior, or store room attendant more often than not. Mech bays filled with stripped mechs. Even even if not, i would have to strip mechs to get things loaded out and look for modules, and engines, or weapons ect...
My new clan account, i have been spending more time just playing, and getting things loaded up with modules/weapons ect and have had a much better time in that since, even if i only have a couple mechs to play. (though the stupid mech sale at the new year ate up some cash that i would of used on modules, but the savings was too great to pass up on a couple mechs, but honestly i haven't even touched um yet.
Next for you
I would buy some mech and weapon modules for your HBK's, grab a seismic sensor, as they are used in so many mechs, Even spend an extra 6m and grab a spare if you like it is really nice to have 2 mechs fully loaded out with modules and ready to go.
I'd really recommend a hill climb module too. Your HBK's really shine with that installed It makes a big difference climbing hills and with no JJ's they really help with getting around. You will find many places on that map that you could not go over with out it installed. After getting used to it i would not run an HBK without it (though i could say the same about many lights and mediums with out JJ's) HBK's have 3 module slots you have room to spare for it.
Target Info gathering is a great module for your 4P, and 4G, It will help help you get the paper doll up quicker to make mode informed choices. Mediums often really benefit from it, as they are more hit and run types, so that extra speed of gathering can help you land a shot in a place that will do the most damage. Hold your fire, a bit longer to get to that open torso, or maybe they have an open back, so save that ac20 slug till you get around back, or flank, and go for the kill instead of just knocking off more armor.
The 4SP on the other hand as the SRM's spread a lot despite having 4 or 5ML's you tend to brawl more in that mech looking away quick, as you recharge for another volley, so may i recommend a 360 targeting module instead for that. It helps you maintain your lock so friendly LRM's can land, or just keep your lock for your lasers.
Lastly i would pick up radar dep, though i often don't use the module over others, the HBK's with it's many slots, .. So that right there is about 24m in modules alone.. but well worth every penny spent. Really get to learn your mechs, and how they play.
But even going with the typical Seismic+ radar dep, that leaves you a slot to play around with trying hill climb, info gathering and 360 lock anyway.. Mix and match and see what suits you best
i'd also pick up some weapons modules, Medium laser range and recharge, along with AC20 recharge, (and i am not sure if you run SRM 4's or 6's in your 4SP, but i would get that module too) Personally i find SRM4's work way better, as you can actually save 4 tons, 2 by not having artemis, and 2 for the lower weight launchers, and with those 4 tons, you can pack in 4 more double heat sinks, in a mech that runs very hot, it is well worth the change.
all that is probably about 30m worth of modules, maybe a bit more, but if you are really enjoying those mechs.. by the time you do all that, my guess is you will really have a great handle on what you are liking, and a much better idea of what weapon systems, and play styles fit you best. HBK's can be brawlers, flankers, or even longer range mechs, so play the heck outta um, and get every cent's worth.
When i started playing my HBK's i played 450 in my 4G, and 300+ in my 4SP before i really started to thing about something new.