200 Subs in a 4 Month period is very good, however here's a few tips:
1 - Try doing reviews on the latest products and before anyone else, this can include doing on the latest Android or iOS or on the latest phones from Sony, Samsung or iPhone, try reaching out to companies for review samples.
2 - Look at your analytics and try and see what videos are getting you the most exposure, not just views but shares, watch time, likes etc.
3 - Try sharing your reviews with Official or Fan pages on Facebook and Twitter for Sony, Samsung, Apple, Android, also getting apart of a community like YTTalk that is specific to tech people is a good start.
4 - Try doing Budget PC Builds if you have the knowledge, and if you have the cash, possibly even buy the parts and build them in the video, these can get very popular with young gamers looking to have a custom PC without breaking the bank, and your video can show them just that. I recommend if you do these do one for every rough price point every 2-5 months.
Hope these help.