The ranges are very close together. I decided to go for a fast survivalist build on my 4P this time. The 6 MPLas, 2 SLas would still kick its ass, but only if that mech can close the distance. Can stay at 400 m range. Good for alpha striking. No need for splitting the weapons up. Group 1: Alpha Strike Group 3: (middle mouse) Chain fire [Change zoom in, zoom out to scroll wheel to open up this weapon group.] If you do not care for the range build, and want something more lethal by all means help yourself! : hbk-4p-expert-6x-mplas-2x-slas-std250 Skill Tree info: LINK!
You're cutting way too much armor on the legs imo, ATMs, lurms and small mechs will destroy you. My edit.
70 kph is plenty to stay at 400m, loss of cooling is a bigger problem probably. Here's a version with std engine (though you can also take the base build and transfer from left arm to legs.